osd: fix map advance limit to handle map gaps

The recent change in cf25bdf6b0090379903981fe8cee5ea75efd7ba0 would stop 
advancing after some number of epochs, but did not take into consideration the
possibilty that there are missing maps.  In that case, it is impossible to
advance past the gap.

Fix this by increasing the max epoch as we go so that we can always get beyond
the gap.

Signed-off-by: Sage Weil <sage@inktank.com>