For persistant processes with some immutable metadata (riak vnode and
the vnode ID, for example), implement lager:md/0 and lager:md/1 for
getting/setting such metadata into the process dictionary.

Such metadata is automatically included in any lager message metadata,
so you can just set it in your init() function or whatever and not have
to worry about passing the data around and using it in every lager call.

Originally an idea that Dizzy forwarded to me from Tony Rogvall.
Basicially all the work lager does gathering metadata and stuff is
wrapped in a case statement that fails-fast if the message is definitely
not going to be logged; the severity isn't currently being consumed and
there's no traces installed.

In my simple test, logging 1 million debug messages, when the debug
level is not being consumed, goes from taking 2.99 seconds to 1.21
seconds with this change.

Also, lager pre 1.0 actually had something similar to this, but it was
removed during a refactor and never re-added.

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.

Additional changes and formatting by Andrew Thompson

az558