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/
Switch voms-api version to 2.0.6

We are using an unsupported and buggy voms-api version

RELEASE NOTES: Switch to voms-api to 2.0.6 fixing "Certificate verification: Maximum certification path length exceeded." exception problem

Patch: <ReviewBoard URL>
Require-notes: yes
Require-book:  no
Bug: http://rt.dcache.org/Ticket/Display.html?id=7848
Acked-by: <name of person>
Target: master
Request: 2.2
Request: 2.6
Merge-req:

The problem I have, I can not test the solution myself as it only happens with certain proxy certificates, see bug ticket for details.

I have however tested that the update does not break g2 and s2 while I did not test xrootd as I have no idea about it.

G2 testing: http://svn.dcache.org/build/view/emi_certification/job/g2-Certification_oneMachine/243/,

mvn building fine, while testing switched on,

S2 test went through fine except for one test, which I would see as a typical S2 timeout problem: http://svn.dcache.org/build/view/emi_certification/job/S2-certification_oneMachine/21/