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.

Implement a new config option error_logger_hwm, which is a number
representing how many messages per second we should log from the
error_logger. If that threshold is exceeded, messages will be discarded
for the remainder of that second.

This is only effective if lager itself can process the messages fast
enough to satisfy the threshold. If your threshold is 1000 and lager
itself is only writing 100 messages a second (because error messages are
causing fsyncs or whatever) you'll never exceed the threshold and drops
will never happen. Thus care needs to be taken when selecting this
feature.

Setting it low is not as bad as it might seem, because when using lager,
hopefully error_logger messages are unusual. In my testing, 50/second
with the default config seemed reasonable (which has 2 file backends
installed, both of which fsync on messages at error or above).

In normal operation, there's no need for log messages to be synchronous.
Its slower and introduces a global serialization point in the
application.

However, when in an overload condition, synchronous logging is good
because it limits each process to at most 1 log message in flight.

So, this change allows the gen_event at the core of lager to switch
modes depending on the size of the gen_event's mailbox. It should
provide better performance in the case of normal load, but it will also
prevent unbounded mailbox growth if an overload occurs.

The threshold at which the switch between async and sync is done is
configured via the 'async_threshold' app env var.

Add add_trace_to_loglevel_config and update_loglevel_config.  These two
handle most of the updates to the loglevel config item, including the
update of the overall logging mask.

This makes minimum_loglevel private.

This doesn't change any behavior -- it just a tidy-up.

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.

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