Move RTPP_NSTATS into rtpp_stats.h, fix index used for accessing
elements in the rtpp_stats_obj_updatebyidx_internal().
Fix two issues with hash_table object initialization.
Implement nstr method in the rtpp_stats object, which returns
string representation of the named counter.
o Implement a new command "G" to get values of the named stats
counters, i.e.:

G ncmds_succd ncmds_errs
2123 5

o extend rtpc_doreply to take extra argument to indicate if it's
normal call completion or error and bump respective counter
accordingly.

o parametrize maximum number of arguments that we are parsing
into RTPC_MAX_ARGC define.
Add rtpp_command_stats.[ch].
Re-gen.