Fix gh-897 - treeCopyFile blocking dafilesrv
Fix gh-897 - treeCopyFile blocking dafilesrv

dafilesrv's throttling mechanism remained blocked if treeCopyFile decided it
was busy and introduced it's own delay.
Consequently a series of treeCopyFile operations would fill up dafilesrv's
throttle pool and all operations would be blocked until treeCopyFile
operations were done.

Change so that when treeCopyFile operations delay (are doing nothing), the
throttle it consumed is temporarily released.

Signed-off-by: Jake Smith <b20121000db38205734e8ca1c74fe9668b08552a@lexisnexis.com>

afishbeck/HPCC-Platform,afishbeck/HPCC-Platform,afishbeck/HPCC-Platform,afishbeck/HPCC-Platform,afishbeck/HPCC-Platform,afishbeck/HPCC-Platform