mon: decode old PGMap Incrementals differently from new ones

mon: decode old PGMap Incrementals differently from new ones

We need to distinguish between the old 0 (meaning undefined) and
the new 0 (meaning switch to 0 and disable the flags). So rev the
encoding version on PGMap::Incremental, and if you decode an old
version with [near]full_ratio == 0, set the ratio to -1 instead. Then
when applying the Incremental interpret -1 as no change.

Signed-off-by: Greg Farnum <gregory.farnum@dreamhost.com>
Reviewed-by: Sage Weil <sage@newdream.net>