Hide AbstractLeader maps

Hide AbstractLeader maps

Collections should never be leaked, as they introduce the ability to
make unsynchronized changes. This patch hides the map itself, allowing
lookups.

Once we have done that, we have control over the interactions and thus
can see that the map is instantiation-constant. Use an ImmutableMap to
maintain the entries as it provides superior compactness and lookup
speeds.

Change-Id: I552372dbabe454111909bb8251637e9f3c0e0db7
Signed-off-by: Robert Varga <rovarga@cisco.com>