"ra systems" are not compatible in a mixed version cluster

[HoloRin]

ra_server_sup_sup.erl performs rpc calls that assume the new version of api introduced in #200

ra/src/ra_server_sup_sup.erl

Line 46 in 8cdbe87

rpc:call(Node, ?MODULE, start_server_rpc, [System, UId, Config]).

Thus, all members must use the same api version and rolling updates across the boundary won't be possible.

[HoloRin]

While a ra cluster won't form across the version boundary, given a quorum of either version, that quorum should work normally. So I will close this as it seems to be a reasonable and unavoidable tradeoff for changing the api fo 2.0.