Remove stale optimization for compaction commits

Before the new compactor we hacked the fsync options to only commit once
before the header was written instead of before and after. This stale
override then lead to an issue where the final commit_data call didn't
fsync after a #db_header{} was written leaving a #comp_header{} as the
last valid header which lead to errors about corruption when opening the
database later on.
Allow processes to idle before exiting

When system load exceeds the ability of os_process_soft_limit to keep
up with demand we enter a fork-use-kill (FUK) cycle. The constant
spawning and destruction os these processes thrashes system resources
and causes general instability.

This patch changes the behavior from killing each process as its
returned to letting it idle for a configurable amount of time (default
five minutes) which allows it to be reused by other clients. This way we
can avoid adding unnecessary load when demand for couchjs processes
exceeds os_process_soft_limit.

As a happy benefit this should also allow os_process_soft_limit to be
set much lower since the number of processes will now more closely
follow actual demand (instead of provisioning for the worst case
scenario).
Add a code_change for addition of 'idle' to State