WL#5797: Move sql/rpl_* to sql/rpl/ and remove the rpl_ prefix

This is a step towards making replication a library, such that the 5.6 rpl-lib can 
be plugged into the 5.5 core [WL#5675].

We will move all replication files to a new subdirectory of sql:
 - Create a new subdirectory of sql/, sql/rpl/
 - Move all replication files from sql/ to sql/rpl/
 - Remove the rpl_ prefix from these files.