Race condition causes fscoordinator crash [JIRA: RIAK-2378]

[ian-mi]

The fscoordinator process can occasionally crash due to an exception such as the following:

exception exit: {{normal,{gen_server,call,[<18203.16286.3>,cluster_name,120000]}},[{gen_server,terminate,6,[{file,"gen_server.erl"},{line,744}]},{proc_lib,init_p_do_apply,3,[{file,"proc_lib.erl"},{line,239}]}]}

Which appears to be triggered by the following call to riak_repl2_fssource:cluster_name: https://github.com/basho/riak_repl/blob/develop/src/riak_repl2_fscoordinator.erl#L850. This is called after attempting to start an fssource when the fssource_sup returns already_started. For this call to trigger a normal exception, I suspect that the fssource must be exiting with normal while the call is in its message queue.