Try to spend the least amount of time handling logging in gRPC thread
executors: do not log to standard output and standard error channels if
'--no-print' is specified (for use in production).

This patch allows filtering the log records that get printed to stdout
and stderr based on their domain name using the BGD_MESSAGE_DEBUG
environment variable. A comma separated list of domains is expected:

  BGD_MESSAGE_DEBUG="buildgrid.server.cas,buildgrid.client.cas"