This is to suppress dialyzer's `unmatched_return` warning

* If the shaper is in overload and the final message comes in, but no
  further messages arrive for some time, until another message came in,
  the drop count would not be printed. Now we set a timer to ensure it
  prints the drop count after the current second expires.
* Allow the shaper to take a filter function that allows events that
  would not normally be printed anyway to not be counted against the
  HWM. This means that if you're suppressing supervisor startup messages
  you won't see drop events counted for messages you'd never see
  printed.

Add ability for handlers to 'fatally' fail, so they won't attempt to
reinstall themselves later.

Also fix an issue with the INT_LOG macro not doing the right thing when
only the lager_throttle_backend was installed.

Adds transparent event stream processing and statistics.
A new 3-tuple trace is introduced as `{Key, Op, Value}`, but
for backwards compatibility `{Key, Val}` implies `=` for `Op`
and `{Key, '*'}` remains as is in the case of wildcards.
A simplified query tree module is generated which reduces
redundant selection conditions to minimize filtering overhead.

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.