Add Pipelining support in psql

Hi,

With \bind, \parse, \bind_named and \close, it is possible to issue
queries from psql using the extended protocol. However, it wasn't
possible to send those queries using pipelining and the only way to
test pipelined queries was through pgbench's tap tests.

The attached patch adds pipelining support to psql with 3 new
meta-commands, mirroring what's already done in pgbench:
- \startpipeline starts a new pipeline. All extended queries will be
queued until the end of the pipeline is reached.
- \endpipeline ends an ongoing pipeline. All queued commands will be
sent to the server and all responses will be processed by the psql.
- \syncpipeline queue a synchronisation point without flushing the
commands to the server

Those meta-commands will allow testing pipelined query behaviour using
psql regression tests.

Regards,
Anthonin