BPlusTree: Meaningless AssertionFailedException if the lock was not acquired

[csharptest]

If the CallLevel lock cannot be acquired due to timeout then the Assertion is triggered. In this case it would be better to throw some specific exception derived from TimeoutException, so the user code can react appropriately.

There are a couple of lines in the constructor of RootLock:

_locked = _exclusive ? _tree._selfLock.TryWrite(tree._options.LockTimeout) :   _tree._selfLock.TryRead(tree._options.LockTimeout);
Assert(_locked);

Instead I suggest to do this:

_locked = _exclusive ? _tree._selfLock.TryWrite(tree._options.LockTimeout) : _tree._selfLock.TryRead(tree._options.LockTimeout);
if (!_locked)
    throw new BPlusTreeTimeoutException("...");

[csharptest]

Fixed on /dev