cells: Fix deadlock during startup

The patch fixes a regression introduced in 2.8. The regression may cause a
deadlock in cells if they receive a message during startup.  This may happen
when restarting individual services in a busy dCache. It is unlikely to happen
when "cold starting" dCache.

The problem is caused by AbstractCell running most cell initialization on the
cell message thread (to ensure the correct context is used). If a message is
received before this task is created, the message will block the message thread
waiting for the cell startup to complete, however the cell startup cannot
complete because the initialization task cannot be executed due to the message
thread being blocked.

The patch solves this by returning a NoRouteToCellException when receiving a
message before cell startup has completed. Thus the blocking behaviour is
removed and the deadlock is avoided.

An unused shutdown gate is removed too.

Target: trunk
Require-notes: yes
Require-book: no
Request: 2.10
Request: 2.9
Request: 2.8
Acked-by: Paul Millar <paul.millar@desy.de>
Patch: https://rb.dcache.org/r/7187/
(cherry picked from commit 61d7402e213a3b438f68860a000d73e9948c7836)
Merge pull request #734 from gbehrmann/fix/2.8/rb7187

cells: Fix deadlock during startup