[2/3] Start a rexi_server per remote node

Switch sending rexi messages to the rexi_server handling requests for
the local node. This replaces the use of the singleton rexi_server
process with a rexi_server instance for each node.

The only slight gotchya in the switch is that we need to send kill
messages to both the per-node and singleton instances during the
transition because we can't tell which process may have generated the
job. The extra kill message will be removed in the next commit.
Merge '[2/3] Start a rexi_server per remote node'

BugzID: 18215