Pass 'log' and 'config' as options to the scrypt module.
Make scrypt.maxPending a config option.

This also renames some internal uses of "max_pending" to match the
prevailing capitalization style.
Periodically log statistics on scrypt queue backlog.
Log a warning when scrypt.maxPending limit is exceeded.
Refactor stat reporting to use opaque methods on each service.
Merge pull request #819 from mozilla/rfk/config-scrypt-backlog

Improve operational affordances for scrypt max-pending limit