mon: OSDMonitor: work around a full version bug introduced in 7fb3804fb

In 7fb3804fb860dcd0340dd3f7c39eec4315f8e4b6 we moved the full version
stashing logic to the encode_trim_extra() function.  However, we forgot
to update the osdmap's 'latest_full' key that should always point to
the latest osdmap full version.  This eventually degenerated in a missing
full version after a trim.  This patch works around this bug by looking
for the latest available full osdmap version in the store and updating
'latest_full' to its proper value.

Related-to: #5704
Backport: cuttlefish

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