nat: static mappings in flow hash

nat: static mappings in flow hash

Put static mappings in flow hash, drop existing hash tables used for
static mappings. Drop refcount variables and use hash table as a single
point of truth. Allow creating a static mapping conflicting with dynamic
mapping, which will take precedence after dynamic mapping is freed, so
that the existing flow can finish transferring data.

Type: fix
Signed-off-by: Klement Sekera <ksekera@cisco.com>
Change-Id: Ieeba691d83a83887d0a0baccd5f3832f66126096