PATCH: regular logging of checkpoint progress

Hello,

I'd like to propose a small patch that allows better checkpoint progress
monitoring. The patch is quite simple - it adds a new integer GUC
"checkpoint_update_limit" and every time checkpoint writes this number of
buffers, it does two things:

(a) logs a "checkpoint status" message into the server log, with info
about total number of buffers to write, number of already written buffers,
current and average write speed and estimate of remaining time

(b) sends bgwriter stats (so that the buffers_checkpoint is updated)

I believe this will make checkpoint tuning easier, especially with large
shared bufferers and large when there's other write activity (so that it's
difficult to see checkpoint I/O).

The default value (0) means this continuous logging is disabled.

Tomas