implement TCP push-pull state sync

[pshemass]

from memberlist paper https://arxiv.org/pdf/1707.00788.pdf

memberlist adds an anti-entropy mechanism, by which each member periodically does a full state sync with another randomly selected member, over TCP. The push- pull approach from [12] is taken, with incarnation num- bers used to reconcile conflicting state about a given member. This full state sync increases the likelihood that nodes are fully converged more quickly, at the expense of more bandwidth usage. It is particularly helpful for speeding up recovery from a network partition.