Have the aae fullsync worker handle all socket operations

[bsparrow435]

Currently socket ownership is passed off to the riak_kv_index_hashtree worker for the vnode being synced during the tree exchange step of aae fullsync. To be less confusing, the callbacks should be handled by the aae worker.

[cmeiklejohn]

Scheduled for 2.1, as part of work to improve AAE-based replication.