mon: OSDMonitor: update the osdmap's latest_full with the new full version

We used to do this on encode_full(), but since [1] we no longer rely on
PaxosService to manage the full maps for us.  And we forgot to write down
the latest_full version to the store, leaving it in a truly outdated state.

[1] - 7fb3804fb860dcd0340dd3f7c39eec4315f8e4b6

Fixes: #5704
Backport: cuttlefish
Signed-off-by: Joao Eduardo Luis <joao.luis@inktank.com>
Reviewed-by: Sage Weil <sage@inktank.com>
mon: OSDMonitor: get rid of encode_full() as we don't use it.

We have delegated this to encode_trim_extra() since
7fb3804fb860dcd0340dd3f7c39eec4315f8e4b6 -- no need to keep this code
around.

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