cells: Fix deadlock

Addresses a regression introduced in 45850f5d50d867772d91b8526d9b7210ae9aaa55. The
regression introduces a deadlock caused by low-level class loading logic in AspectJ
using the logging system, and our logging system looks up cells through getCell(),
a method made synchronized by the above commit.

The solution adopted is to not have two cell maps. Rather we have a set of killed
cells to track the fact that a cell was killed.

Target: trunk
Request: 2.6
Request: 2.2
Require-notes: no
Require-book: no
Acked-by: Paul Millar <paul.millar@desy.de>
Patch: http://rb.dcache.org/r/5718/