Add record to central session/user lookup table slow down sessions per second performance for multi thread setup.
Move following from snat_maint_t:
/* Main lookup tables */
clib_bihash_8_8_t out2in;
clib_bihash_8_8_t in2out;
/* Find-a-user => src address lookup */
clib_bihash_8_8_t user_hash;
to snat_main_per_thread_data_t