1. Add set_high_water/1 to adjust the high water mark after startup
2. Add test func t0/0 to demo (interactively only, sorry) that the
   limiting is working as we expect).
3. Remove a couple of comments.

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.

Don't assume the supervisor is a registered process.

This fixes a bug where messages were incorrectly discarded if the new
handler is using log levels that are different from those in use by handlers
that were set up at start-of-day.

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.

Improve lager_console_backend to support non-atom output log levels.

This supports forms such as "!notice" for configuring the output
log level (using the config handling changes from 7aa316902e).

Lager doesn't print iolists with ~s right

Parse transform can't handle `Attr' as a variable

Empty registered processes section in lager.app.src.

As long as *one* of the arguments is a literal, lager can figure out
what you're trying to do and (re)arrange the arguments as necessary.

Tracing without using parse transform.

add whitelist of error_logger handlers not to delete when error_logger_r...

This helps systools detect registered name clashes

Support pretty printing records found in a module at compile time

Support backend modules defining their own gen_event handler ID

Reject invalid format strings more aggressively

BUGFIX: Avoid overloading the LOG macro.

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

Added node to metadata in parse transfrom