enumerable_thread_specific: remove volatile hash map

Now, thread id of each thread is also an index to persistent storage
for enumerbale_thread_specific (this index is the same for all
instances of enumerable_thread_specifc which certain thread uses).

This change means that persistent storage can be bigger than neccesary
in some cases (it must be as big as largest 'thread id + 1'). This
implementation also allowed for removal of read lock in local() method.
Now, it's enough to check atomic size variable and if size < thread id
just return element with this index.

This commit also removes local(bool) method beacuse it's not possible to
tell whether certain element existed before or not.