KAZOO-2538: track last-accessed timestamp for token bucket

cleanup buckets not used recently (defined in system_config)
KAZOO-2538: add inactivity config value
KAZOO-2538: token bucket servers should not be restarted
KAZOO-2538: bucket server should check for inactivity and stop
themselves after inactivity threshold is reached.
KAZOO-2538: add some more time-related macros
KAZOO-2538: extend elapsed_* functions to be callable with both 'start'
and 'now' parameters
Merge pull request #371 from 2600hz/KAZOO-2538

Kazoo 2538