Fix timestamp of sent message for write context in logical decoding

When sending data for logical decoding using the streaming replication
protocol via a WAL sender, the timestamp of the sent write message is
allocated at the beginning of the message when preparing for the write,
and actually computed when the write message is ready to be sent.

The timestamp was getting computed after sending the message.  This
impacts anything using logical decoding, causing for example logical
replication to report mostly NULL for last_msg_send_time in
pg_stat_subscription.

This commit makes sure that the timestamp is computed before sending the
message.  This is wrong since 5a991ef8, so backpatch down to 9.4.

Author: Jeff Janes
Discussion: https://postgr.es/m/CAMkU=1z=WMn8jt7iEdC5sYNaPgAgOASb_OW5JYv-vMdYaJSL-w@mail.gmail.com
Backpatch-through: 9.4