bdr: mergme: cleanups for contrib/bdr by alvaro
bdr: mergme: fix accidental closing of -1 fd
wal_decoding: bdr: mergme: Alvaro's fixes for reorderbuffer.c
bdr: isolationtester: Isolationtester with multi-server support

The spec file can now start with declarations like this:

    conninfo "cname1" "port=NNNN dbname=XYZ"
    conninfo "cname2" "port=MMMM dbname=PQR"

And sessions may include connection declarations like this:

    session "s1"
    connection "cname1"

Both are optional. If no connection information is explicitly specified,
then the earlier behaviour (use conninfo from commandline or fall back
to "dbname=postgres") is retained.

If connection information is specified, we open one "global" connection
per conninfo (for lock checking) and one connection per session. Setup
SQL is executed only on conns[0], as before (but perhaps it would help
to execute it on all until DDL replication is implemented?).

Includes the rudimentary tests from before.
bdr: isolationtester: test#1: Add an UPDATE/DELETE conflict (breaks)
bdr: mergme: Optionally lock tuple when looking it up during apply.
Run global setup/teardown on all given conninfos

This saves us a lot of bookkeeping in the session setup to create the
required tables and functions on all nodes.
A revised set of tests