Slaves list in INFO output: lag added, format changed.

There is a new 'lag' information in the list of slaves, in the
"replication" section of the INFO output.

Also the format was changed in a backward incompatible way in order to
make it more easy to parse if new fields are added in the future, as the
new format is comma separated but has named fields (no longer positional
fields).
repl_offset field in INFO replication is now just offset.
min-slaves-to-write: initial description of the feature in redis.conf
min-slaves-to-write: don't accept writes with less than N replicas.

This feature allows the user to specify the minimum number of
connected replicas having a lag less or equal than the specified
amount of seconds for writes to be accepted.
min-replicas-to-write: only deny write commands.

I guess I needed another coffee...
Make tests compatible with new INFO replication output.
Refresh good slaves count when setting slave state as online.
New INFO field "min_slaves_good_slaves".

When min-slaves-to-write feature is active, this field reports the
number of slaves considered good (online state, lag within the specified
range).
Refresh good slaves count after CONFIG SET min-slaves-...

This way just after the CONFIG SET enabling the min-slaves feature it is
possible to write to the database without delays.
Tests added for min-slaves feature.
Test: avoid a false positive in min-slaves test.
CONFIG SET: accept slave-priority zero, it is valid.
Binary safe dump of object content in redisLogObjectDebugInfo().