mon: fix read of format_version out of leveldb

The get_version(string, string) is the wrong method; it combines the two
args into a key that is nested inside prefix (so it's prefix/a/b), but we
want perfix/format_version.  Add a method to grab an int for this
particular combo and use that.

This fixes an infinite loop when we actually trigger this code.

Bug introduced by f43c974571beac0c8e54fa699bfa96a1befaf56c.

Signed-off-by: Sage Weil <sage@inktank.com>
Reviewed-by: Joao Eduardo Luis <joao.luis@inktank.com>