Concurrent access after removal of an AtomicMap should NOT result in an IllegalStateException when accessed by other threads

ISPN-1121 introduces an IllegalStateException that is being thrown from AtomicMap methods once the map handle has become stale (ie. removed from cache). In case of concurrent access, the exception is thrown to all threads not just to the thread that performed the removal. This was fine-ish in older versions of Infinispan before introduction of optimistic and pessimistic locking but it should be reconsidered now because:

1. It interferes/overlaps with transaction isolation. We should rely on the selected locking scheme (OL/PL) to detect conflicts between transactions and report the problem accordingly. Especially if the stale map is used just for reading - this should be allowed to work rather than stop it.
2. This exception is pretty disruptive and awkward to handle. All methods of an AtomicMap can result in this exception and the current thread has to be prepared for handling it if other threads remove the map. Not much transaction isolation.
3. Since the TreeCache is backed by AtomicMap nearly all Tree API can throw this.

The proposed fix consists of:
1. removing AtomicHashMap.removed flag and AtomicHashMap.markRemoved() method.
2. revising AtomicHashMapProxy.assertValid() method to check only if the map is null (ie. removed) but no longer use the removed flag.
3. revising ReadCommittedEntry.commit() method to no longer call markRemoved() method.

The consequences of these changes are:
1. Any further access to a stale map results in IllegalStateException ONLY in the thread that performed the removal. This thread 'knows' the map is stale so it is fine to punish it. Other threads remain unaffected until lock acquisition or commit is performed (depending on locking model).
2. Other threads can continue to use the previously obtained map handle for reads without danger of getting an exception.
3. If a write operation is done on the map, the results depend on the locking model:
3.1 optimistic locking + write skew check: a WriteSkewException will stop the commit during prepare
3.2 optimistic locking, no write skew check: the write is committed and the work of the transaction that removed the map is overwritten. The map is effectively revived.
3.3 pessimistic locking: same as 3.2

Please note 3.2 and 3.3 work the same as for normal values (not atomic maps).