Previously the "Show Monthly Usage" command showed the stats that
Catapult recorded, which could sometimes over-represent one's usage
(see 5e609a45, 0e664811, and 76d31968 for the original implementation).

Now we instead show the stats that are recorded by jmp-fwdcalls and
sgx-catapult, which were added in 09f98c8f and
sgx-catapult@31c2cb7c
, respectively.  These stats are saved as integer values of the
usage_minutes- and usage_messages-[TAI_date]-[JMP_number] keys.

Although this does complete the implementation for jmp-fwdcalls, the
code added here does rely on the new usage_start_day-[JMP_number] keys
and the values are not yet generated by jmp-register (nor have they
been manually populated in production databases where numbers have
already been added without their respective usage_start_day-* values).

Until that is done, one will receive "Unable to get usage statistics"
when running the "Show Monthly Usage" command, which is the standard
message if jmp-fwdcalls cannot determine a JMP number's start date.