osd: prevent pgs from getting too far ahead of the min pg epoch

osd: prevent pgs from getting too far ahead of the min pg epoch

Bound the range of PG epochs between the slowest and fastest pg
(epoch-wise) with 'osd map max advance'.  This value should be set to
something less than 'osd map cache size' so that the maps we are
processing will be in memory as many PGs advance forward in time in
loose synchrony.

This is part of the solution to #7576.

Signed-off-by: Sage Weil <sage@inktank.com>
(cherry picked from commit cf25bdf6b0090379903981fe8cee5ea75efd7ba0)