When a connection is closed, we also need to remove the hash entry
from the regular_list, as it now points to freed memory. To do this
store a reverse mapping from the connection to the hash string.

It would be nicer to introduce a wrapping structure for the pgsql
link resource that could store the hash (and notices), but that would
require large changes to the extension, so I'm going for a more
minimal fix here.

The $Id$ keywords were used in Subversion where they can be substituted
with filename, last revision number change, last changed date, and last
user who changed it.

In Git this functionality is different and can be done with Git attribute
ident. These need to be defined manually for each file in the
.gitattributes file and are afterwards replaced with 40-character
hexadecimal blob object name which is based only on the particular file
contents.

This patch simplifies handling of $Id$ keywords by removing them since
they are not used anymore.

Also re bug #71575.