Make pending_timeout an argument of __init__

Just like stale_reads_ok, this was already an argument, end even
used by the replicator that way. We only make it used exclusively
this way, so that we can document it for LFS.

Change-Id: Ib722093a476ec75d87917b37b40b77dcf2627330
Merge "Make pending_timeout an argument of __init__"