Changes

add hnb_persistent hashtable: optimize lookup by cell id

Mainly the new nft counters do a lot of hnb_persistent lookups by cell
id.

Add a hashtable to optimize looking up hnb_persistent instances. So far
we iterate the linear list of hnb_persistent for each and every counter
returned from nft_kpi.c.

Also improves lookups for HNBAP HNB* operations (rare).

A follow-up patch uses a better hash function from libosmocore
(jhash.h).

Related: SYS#6773
Change-Id: Iecb81eba28263ecf90a09c108995f6fb6f5f81f2