Today we use a sliced lock strategy for acquiring locks to prevent
concurrent updates to the same document. The number of sliced locks is
computed as a linear function of the number of logical
processors. Unfortunately, the probability of a collision against a
sliced lock is prone to the birthday problem and grows faster than
expected. In fact, the mathematics works out such that for a fixed
target probability of collision, the number of lock slices should grow
like the square of the number of logical processors. This is
less-than-ideal, and we can do better anyway. This commit introduces a
strategy for avoiding lock contention within the internal
engine. Ideally, we would only have lock contention if there were
concurrent updates to the same document. We can get close to this ideal
world by associating a lock with the ID of each document. This
association can be held in a concurrent hash map. Now, the JDK
ConcurrentHashMap also uses a sliced lock internally, but it has several
strategies for avoiding taking the locks and these locks are only held
for a very short period of time. This implementation associates a
reference count with the lock that is associated with a document ID and
automatically removes the document ID from the concurrent hash map when
the reference count reaches zero.

Relates elastic#18060