Allow configuration of log destination

Look for LOG_FILE configuration directive to control the destination
stream for log events.

Change also adds several Monolog processors that will decorate all log
events with information about the active request and installs the
Logstash event formatter. The FingersCrossedHandler that was previously
configured has also been removed.

Bug: 57692
Change-Id: I9529988af2a1282a64e1ec8c4d15d0e665e5014e
Merge "Allow configuration of log destination"