Refrain from duplicating data in reorderbuffers

Refrain from duplicating data in reorderbuffers

If a walsender exits leaving data in reorderbuffers, the next walsender
that tries to decode the same transaction would append its decoded data
in the same spill files without truncating it first, which effectively
duplicate the data.  Avoid that by removing any leftover reorderbuffer
spill files when a walsender starts.

Backpatch to 9.4; this bug has been there from the very beginning of
logical decoding.

Author: Craig Ringer, revised by me
Reviewed by: Álvaro Herrera, Petr Jelínek, Masahiko Sawada