Sometimes there are a LOT of records going through logging backends
which contain notable number of empty (i.e. undefined) fields. This
humble patch solemnly introduces new function `lager:pr/3` designed to
help cope with such kind of issues, providing a one and only option `compress`
which, when turned on, effectively eliminates any empty fields in records.

Finish lager API modifications
Update config API to handle multiple sinks
Identify areas where new code is yet to be written

First pass at adding sink parameter into the external API layer.

TODO:
* config updates
* loglevel changes per sink

Improve format_reason for certain oddly formed badarg stacktraces

Typos, add formatting, rework the semi-iolist description

add per-module per-function trace example

Fix/make trace stop trace api consistent cleanup

Add optional log_root environment variable

Mention the color atom for the formatter config

Fix specification of backend config in log rotation example

add application stop/start suppression for ct logs

Reviewed-by: seancribbs

in common test, starting and stopping applications can make your logs
very noisy.  the patch allows these messages to be suppressed on the
happy path, and raises the high water mark during ct tests so that
messages aren't dropped.