mon: MonmapMonitor: support 'mon getmap [epoch]'

Fixes: #6683

Signed-off-by: Joao Eduardo Luis <joao.luis@inktank.com>
mon: OSDMonitor: proper error msg on invalid epoch on 'osd getmap/dump'

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