MINOR: stktable: use {show,set,clear} table with ptr

MINOR: stktable: use {show,set,clear} table with ptr

This patchs adds support for optional ptr (0xffff form) instead of key
argument to match against existing sticktable entries, ie: if the key is
empty or cannot be matched on the cli due to incompatible characters.
Lookup is performed using a linear search so it will be slower than key
search which relies on eb tree lookup.

Example:

set table mytable key mykey data.gpc0 1

show table mytable
> 0x7fbd00032bd8: key=mykey use=0 exp=86373242 shard=0 gpc0=1

clear table mytable ptr 0x7fbd00032bd8

This patchs depends on:
 - "MINOR: stktable: add table_process_entry helper function"

It should solve GH #2118