Avoid unnecessary decoding in ziplist.c

Closes #1519
pubsub: Return integers for NUMSUB, not strings

Also adds test for numsub — due to tcl being tcl,
it doesn't capture the "numberness" of the fix,
but now we at least have one test case for numsub.

Closes #1561
Reject MOVE to non-integer DBs

Previously, "MOVE key somestring" would move the key to
DB 0 which is just unexpected and wrong.
String as DB == error.

Test added too.

Modified by @antirez in order to use the getLongLongFromObject() API
instead of strtol().

Fixes #1428