R1:e846a1747ef3

Fix Feed Times on 32 Bit Servers

Summary:
The feed time is stored as the upper 32 bits of
PhabricatorFeedStoryData::chronologicalKey. These bits were previously accessed
by right shifting, which does not work properly on 32 bit machines (the result
is PHP_INT_MAX). We now attempt to use the bc extension (if available) and fall
back on mysql math otherwise. (See T500, D912).

Test Plan:
The calculation is unchanged for 64 bit machines. I checked both
paths on a 32 bit machine with bc extension available by…