Use rust logging API (pantsbuild#5525)

When in the engine, this passes messages up to python to log.

When in a standalone binary, this registers an env_logger.

Examples:

```
$ RUST_LOG=debug ./target/debug/process_executor --server=localhost:50000 --cas-server=localhost:50001 --local-store-path=${HOME}/tmp/store --input-digest=6f9a33e23a007c58c1e83f29bcf52979c797aba200cec0058695ff72d181e6ed --input-digest-length=85 -- /bin/bash -c "exit 42"
DEBUG 2018-02-27T15:23:19Z: process_execution::remote: Executing remotely request: action {command_digest {hash: "8a1eb3dc8694551ee161e6f616de205f7c2d5b52c8f5b93d2745d67c3b750e6a" size_bytes: 24} input_root_digest {hash: "6f9a33e23a007c58c1e83f29bcf52979c797aba200cec0058695ff72d181e6ed" size_bytes: 85}} (command: arguments: "/bin/bash" arguments: "-c" arguments: "exit 42")
DEBUG 2018-02-27T15:23:20Z: process_execution::remote: Got operation response: name: "b15e0908-7956-4dfa-7fc7-48dffc8aff8d" metadata {type_url: "type.googleapis.com/google.devtools.remoteexecution.v1test.ExecuteOperationMetadata" value: "\010\002\022E\n@81d6ae5c2323235c8633ffa63a1867bcc40306e0da9012a3d387df2eead80e3d\020\214\001"} response {type_url: "type.googleapis.com/google.devtools.remoteexecution.v1test.ExecuteResponse"}
```

```
$ ./pants -ldebug clean-all >/dev/null 2>/dev/null ; ./pants -ldebug list 3rdparty:: 2>&1 | grep Launching
DEBUG] Launching 2 roots.
DEBUG] Launching 1 roots.
DEBUG] Launching 2 roots.
```

src/rust/engine/Cargo.toml

@@ -28,6 +28,7 @@ fs = { path = "fs" }
 futures = "0.1.16"
 hashing = { path = "hashing" }
 lazy_static = "0.2.2"
+log = "0.4"
 ordermap = "0.2.8"
 petgraph = "0.4.5"
 process_execution = { path = "process_execution" }

src/rust/engine/fs/fs_util/Cargo.toml

@@ -8,6 +8,7 @@ bazel_protos = { path = "../../process_execution/bazel_protos" }
 boxfuture = { path = "../../boxfuture" }
 bytes = "0.4.5"
 clap = "2"
+env_logger = "0.5.4"
 fs = { path = ".." }
 futures = "0.1.16"
 hashing = { path = "../../hashing" }

src/rust/engine/fs/fs_util/src/main.rs

@@ -2,6 +2,7 @@ extern crate bazel_protos;
 extern crate boxfuture;
 extern crate bytes;
 extern crate clap;
+extern crate env_logger;
 extern crate fs;
 extern crate futures;
 extern crate hashing;
@@ -37,6 +38,8 @@ impl From<String> for ExitError {
 }
 
 fn main() {
+  env_logger::init();
+
   match execute(
     App::new("fs_util")
       .subcommand(

src/rust/engine/process_execution/Cargo.toml

@@ -12,6 +12,7 @@ fs = { path = "../fs" }
 futures = "0.1.16"
 grpcio = { version = "0.2.0", features = ["secure"] }
 hashing = { path = "../hashing" }
+log = "0.4"
 protobuf = { version = "1.4.1", features = ["with-bytes"] }
 sha2 = "0.6.0"
 tempdir = "0.3.5"

src/rust/engine/process_execution/src/lib.rs

@@ -6,6 +6,8 @@ extern crate fs;
 extern crate futures;
 extern crate grpcio;
 extern crate hashing;
+#[macro_use]
+extern crate log;
 #[cfg(test)]
 extern crate mock;
 extern crate protobuf;

src/rust/engine/process_execution/src/remote.rs

@@ -80,8 +80,7 @@ impl CommandRunner {
       Ok((command, execute_request)) => {
         self.upload_command(&command, execute_request.get_action().get_command_digest().into())
           .and_then(move |_| {
-            // TODO: Log less verbosely
-            println!("Executing remotely request: {:?} (command: {:?})", execute_request, command);
+            debug!("Executing remotely request: {:?} (command: {:?})", execute_request, command);
 
             map_grpc_result(execution_client.execute(&execute_request))
                 .map(|result| (Arc::new(execute_request), result))
@@ -194,7 +193,7 @@ fn extract_execute_response(
   mut operation: bazel_protos::operations::Operation,
 ) -> Result<ExecuteProcessResult, ExecutionError> {
   // TODO: Log less verbosely
-  println!("Got operation response: {:?}", operation);
+  debug!("Got operation response: {:?}", operation);
   if !operation.get_done() {
     return Err(ExecutionError::NotFinished(operation.take_name()));
   }
@@ -213,7 +212,7 @@ fn extract_execute_response(
       ExecutionError::Fatal(format!("Invalid ExecuteResponse: {:?}", e))
     })?;
   // TODO: Log less verbosely
-  println!("Got (nested) execute response: {:?}", execute_response);
+  debug!("Got (nested) execute response: {:?}", execute_response);
 
   match grpcio::RpcStatusCode::from(execute_response.get_status().get_code()) {
     grpcio::RpcStatusCode::Ok => Ok(ExecuteProcessResult {