iproto: fix IPROTO_SERVER_IS_RO key code

IPROTO_SERVER_IS_RO currently has code 0x07 and is defined in the header
key section, which is wrong, because this key is only used in request
body. Let's move it to the body section, where it belongs, and set its
code to 0x29. This shouldn't break anything even if 0x07 is reused in
future, because the two codes belong to different sections and hence are
never parsed in the same function. Worst that can happen is we fail to
bootstrap a node in the cluster if it is running a newer tarantool
version.

While we are at it, let's also add the key name and change its type from
MP_UINT to MP_BOOL.

Fixes commit a8ecd1e1 ("replication: fix bug with read-only replica
as a bootstrap leader").