Use ImmutableMaps for lazy augmentations initialization

Use ImmutableMaps for lazy augmentations initialization

Rather than tracking lookups in two separate ConcurrentMaps, we
perform locked atomic updates in copy-on-write immutable structure.

The two lookup maps are kept in ImmutableMaps, which offer tighter
packing and do not incur any further volatile reads beyond getting
a reference to them.

This results in lower memory overhead, as the maps are more tightly
packed and also biases performance towards the dominant use case,
which is pure lookups.

JIRA: MDSAL-391
Change-Id: I65ecb25a5d364040be44df144000d1bbbc2a922e
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>