Remove server support for the previous base backup protocol.
Commit
cc333f32336f5146b75190f57ef587dff225f565 added a new COPY
sub-protocol for taking base backups, but retained support for the
previous protocol. For the same reasons articulated in the message
for commit
9cd28c2e5f11dfeef64a14035b82e70acead65fd, remove support
for the previous protocol from the server.
Discussion: http://postgr.es/m/CA+TgmoazKcKUWtqVa0xZqSzbKgTH+X-aw4V7GyLD68EpDLMh8A@mail.gmail.com