Rename CHECKPOINT_IMMEDIATE to CHECKPOINT_FAST.

Rename CHECKPOINT_IMMEDIATE to CHECKPOINT_FAST.

The new name more accurately reflects the effects of this flag on a
requested checkpoint.  Checkpoint-related log messages (i.e., those
controlled by the log_checkpoints configuration parameter) will now
say "fast" instead of "immediate", too.  Likewise, references to
"immediate" checkpoints in the documentation have been updated to
say "fast".  This is preparatory work for a follow-up commit that
will add a MODE option to the CHECKPOINT command.

Author: Christoph Berg <[email protected]>
Discussion: https://postgr.es/m/aDnaKTEf-0dLiEfz%40msg.df7cb.de