mon: Monitor: convert osdmap_full as well

Store conversion wasn't converting the osdmap_full/ versions, only the
incrementals under osdmap/ and the latest full version stashed.  This
would lead to some serious problems during OSDMonitor's update_from_paxos
when the latest stashed didn't correspond to the first available
incremental.

Fixes: #4521

Signed-off-by: Joao Eduardo Luis <joao.luis@inktank.com>
mon: add tool to fix store conversion bug manually

This tool is to be used to fix the cause of #4521, and it
will take an old-format store and convert all the osdmap's
full versions to the new-format k/v store.

Fixes: #4521

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