BUG-509: use concurrent hash-trie Map

This patch switches to using concurrent hash-trie map for metadata
tracking. It has the nice feature of having O(1) concurrent snapshots,
which should give us better performance when dealing with large children
bases.

This is a trade-off, please refer to yangtools.util.MapAdaptor for
system properties which tune its behavior.

Change-Id: I2ddb81a1296cc17528a605eeaeda1f303560fb55
Signed-off-by: Robert Varga <rovarga@cisco.com>