BugzID: 11589 call file:close explicitly to keep VM responsive

Deleting a large file in the previous scheme has caused the Erlang VM
to become unresponsive (including making it drop out of the ring of
nodes).

The cause of this is currently unknown but Adam discovered that
explicitly calling file:close/1 on the file descriptor does not cause
this behavior.

Accordingly, I have typed in his suggested fix and tested it.

Conflicts:
	src/couch/src/couch_file.erl
Monitor sys_db file descriptor to keep it open

A couch_file process occasionally checks to see if it is being
monitored by any clients, and closes if not.  Specifically, it checks
for at least two monitoring processes, because it assumes
couch_stats_collector is one of them.  That assumption was wrong
for sys_db files, so they would shut down after a minute and need to
be reopened.

BugzID: 11641

Conflicts:
	src/couch/src/couch_file.erl