Allows fetching custom non-UUID keys when support is enabled (prebid#48)

Allows activating the key setting feature via ENV variables. (prebid#47)

* Allows activating the key setting feature via ENV variables.

* Sets proper default

Allows for providing a key to store caches under (prebid#45)

* Allows for providing a key to store caches under

* Adds comments for possible future enhancements.

* Fixes some logic

* Fixes error handling

Added aerospike expiry metrics (prebid#42)

* Added expiry metrics to aerospike.

* Added a measurement to see how many objects are defining their TTL explicitly/

Added a default max-ttl config. (prebid#41)

Redis backend (prebid#38)

* Redis backend

* Remove logging password

* changes

* consistent naming when logging + change default backend to memory

Swapped the order on metrics/compression. (prebid#35)

Removed the explicit viper.GetString (prebid#34)

Log config values on startup (prebid#33)

* Logged all the config values on app startup.

* Renamed some stuff, and fixed a missing validation step.

Respect environment variables (prebid#31)

* Added environment variable support.

* Added environment variable config support.

* Added an environment variable example to the README.