Refactor fullsync strategy specific decision logic in source and sink

[bsparrow435]

riak_repl2_fssource and riak_repl2_fssink have a bunch of case statements based on Strategy which is either keylist or aae. These case statements should be replaced with a utility function that takes in Strategy and sink or source and returns the necessary module. (ex. riak_repl_aae_source or riak_repl_keylist_server. This would be much cleaner than the current implementation especially for cancel_fullsync calls and other similar handles.

[cmeiklejohn]

Moving to 2.1.