Introduce MapAdaptor.initialSnapshot()

Introduce MapAdaptor.initialSnapshot()

In some circumstances we have an idea about how many entries will there
be in a map. This could cost us perform one additional copy, so expose a
method to create the appropriate map, behaving just as the one returned
by takeSnapshot(Collections.emptyMap()).

Also use ImmutableMaps for empty map, as it being immutable is recognized
by Guava, leading to quick reuse.

Change-Id: I40925e5d643c074277e9d11365a3b10fb9f22c1d
Signed-off-by: Robert Varga <rovarga@cisco.com>
(cherry picked from commit 92fc5cc4436a65c3a63e1fc7afbff2ac42a70880)