csl: Publish externally with pants, sbt, and friends

Problem

It's difficult to publish util, finagle, etc. because we haven't
updated them for the new maven-less world.

Solution

Scripts that make it easy to publish with sbt and pants.  Bumped
the server-set version because we stopped publishing thrift
externally in ci.  Changed an ExitGuard test which broke under
weird sbt conditions.

Result

We're in a position where we can start publishing finagle, etc
again!

RB_ID=633570

Bump all sbt projects (util, ostrich, finagle, twitter-server)

Problem

After looking at the finagle changelog, I discovered that we already had
entries for finagle 6.23.0 and more for the current version.
The previous version bump was then incorrect, the version needs to be
greater than 6.23.0.

Solution

Bump the minor version of finagle, and for consistency I did the same
thing for all the other related projects.
Close the current section of the finagle changelog and name it "6.24.0".

RB_ID=540180

Update sbt projet definitions of (util, ostrich, scoorge, twitter-ser…

…ver)

Problem

In order to release our libraries for the open-source community, we need to update
the version of those.

Solution

Update the version in the sbt project definition.
While I was there, update also sbt to the latest version and increase the jvm
configuration for the code cache.

RB_ID=537603

util version 6.12.1