Allow BACKLOG_TIMEOUT to be configured

[victoriagrey]

We're currently hardcoding the backlog timeout to 5 minutes. While this is a decent default, it'd be best if this were user-configurable.

shotgunEvents/src/shotgunEventDaemon.py

Line 804 in d77a0c1

BACKLOG_TIMEOUT = 5 # time in minutes after which we consider a pending event won't happen