Improved how aliases are handled in the cluster state.

The following changes improved alias creation:
* Moved away from ImmutableMap to JCF's UnmodifiableMap. The ImmutableMap always made a copy, whereas the UnmodifiableMap just wraps the target map.
* Reducing the number of maps being created during the creation of MetaData and IndexMetadata.
* Changed IndexAliasesService's aliases from a copy on write ImmutableMap to ConcurrentMap.

Closes #3410