OPENDJ-2393: Possible index corruption

In case of operation cancellation/exception, transactions are rolled
back and with them the potential update of indexes' trusted state.
That's why in case of transaction rollback we persist the indexes
trusted state using another transaction.

The number of keys contained in a VLV index is cached. This number can
becomes incorrect when transaction updating index keys is aborted. The
number of keys is now persisted using a sharded counted.

Incorrect PDB RollbackException handling could have cause operations
to abort rather than being retried.

Refactoring: fix comments, removed dead code (obsoleted by id2children &
id2subtree removal)

Use specific compact encoding for shared-counter values.