aae and keylist fullsync strategies should use common TCP handling code

[bsparrow435]

The two current fullsync strategies use divergent TCP handling code with the keylist workers leaving socket ownership with the riak_repl2_fssource and riak_repl2_fssink while aae transfers ownership between several worker processes. This is very confusing and detrimental to the clarity of the overall code base.

[cmeiklejohn]

Moving to 2.1.