Atomically collect callbacks to be broken

Atomically collect callbacks to be broken

Collect callbacks to be broken in one go, otherwise a file server may
lock out a client while new callbacks tickle in.

With revised multi_Rx, responses get handled early, taking away an argument for
issuing callbacks in small chunks -> crank up chunk size.

Change-Id: I6822256715d1388aa1a44049315813ea08009105
Reviewed-on: http://gerrit.openafs.org/3909
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@openafs.org>