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.

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

Add "SMTP Support" section to README.org

Unnecessary debug message

Fix custom formatter spec for lager_file_backend (version 2)

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