mds: MDSMap: add encode_plaintext/formatted_summary()

And remove print_summary(), substituting its invocations for the plaintext
version of the newly added functions.

Signed-off-by: Joao Eduardo Luis <joao.luis@inktank.com>
mon: MDSMonitor: formatted output on 'mds stat'

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