Fix race between close() and get() in StoreDirect

[aleksandrserbin]

There is a race, when Store is closed, but there are still iterators somewhere, which may use it.
In the concurrent environment, get() call can pass assertNotClosed assertion, but acquire the lock after Store::close completed.

Probably linked to #892 as well, if there is a scenario with concurrent close and get.

Note: this will still lead to exception, however it will be an IllegalAccessError, rather then NullPointerException