This is done via a combination of several things:
* Make the loglevel that triggers a sync configurable
* Make the delayed_write size and intervals configurable
* Make the interval at which external rotation is checked for
  configurable
* Store the timestamp a lager_msg was created inside the lager_msg

To support these changes, several other things had to be modified:
* lager_msg:timestamp now returns a timestamp
* lager_msg:datetime was added to return the {date, time} of a message,
  like lager_msg:timestamp used to
* The configuration syntax for file backends was changed to be of the
  form {lager_file_backend, proplist()} and the old syntax was
  deprecated

Additionally, the defaults for the check_interval was raised from
'always' to 1 second, and the sync_interval was changed from 2 seconds
to one second.

Also, adapt the rest of lager to use this bitmask as well.

This also removes the dependency on the syntax_tools and compiler
applications, which interfered with lager's startup/shutdown order.

Helps with pattern matching.

I didn't want the lager_log_message record being used across application
boundaries, this will insulate other applications from any changes to
the message type's internal structure.

If backends have a failure or something, don't discard the messages,
wait half a second and then try sending them again.