rgw: rework replica log + RESTful api

rgw: rework replica log + RESTful api

Rather than making the caller code handle encode/decode/dump etc,
we introduce an RGWReplicaBounds struct which the RGWReplicaLogger classes
traffic in (instead of the string marker, utime_t time, and list of pairs of
items). Right now this is just referring to cls_replica_log internal structs,
but we use typedefs so that we can cleanly change them over later and avoid
crossing the namespaces too much.

This greatly reduces the amount of code in rgw_rest_replica_log -- hurrah!

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
Signed-off-by: Greg Farnum <greg@inktank.com>