Disconnect TrieMap from java.util.AbstractMap

[rovarga]

TrieMap is a fully-fledged implementation, overriding pretty much all methods provided by AbstractMap. Furthermore, unlike ConcurrentHashMap, we do not have access to our keySet field, leading to us duplicating it.

Do not extend AbstractMap, implementing all ConcurrentHashMap methods explicitly. Things like hashCode(), equals() and toString() can nicely be implemented in ImmutableTrieMap and having MutableTrieMap create a snapshot and delegate to it.