BUG-7464: Split TrieMap into read-only and read-write

BUG-7464: Split TrieMap into read-only and read-write

Having inter-twined mutable and immutable implementations is
a maintenance and performance drag, because mutable version
is checking for read-only on each mutation and also everytime
it needs to CAS_ROOT. On the other hand, the immutable
implementation does not need to care about RDCSS, as it never
updates its root -- and can in fact have it non-volatile.

Split the implementation into three classes, with TrieMap
definining the API contract and holding common bits, MutableTrieMap
encapsulating RDCSS and mutation bits and ImmutableTrieMap
having simple guards against mutation.

Change-Id: Icae923d6312901b37252af80b03b0a0912cacaec
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>