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>
(cherry picked from commit 1e0a82fd55dede473c0af32924f4bcb5bb697a2b)