`org.elasticsearch.action.bulk.BulkProcessor` is a threadsafe class that
allows for simple semantics to deal with sending bulk requests. Once a
bulk reaches it's pre-defined size, documents, or flush interval it will
execute sending the bulk. One configurable option is the number of concurrent
outstanding bulk requests. That concurrency is implemented in
`org.elasticsearch.action.bulk.BulkRequestHandler` via a semaphore. However,
the only code that currently calls into this code is blocked by `synchronized`
methods. This results in the in-ability for the BulkProcessor to behave concurrently
despite supporting configurable amounts of concurrent requests.

This change removes the `synchronized` method in favor an explicit
lock around the non-thread safe parts of the method. The call into
`org.elasticsearch.action.bulk.BulkRequestHandler` is no longer blocking, which
allows `org.elasticsearch.action.bulk.BulkRequestHandler` to handle it's own concurrency.