Handle CloseComplete Postgres response in _parse_execute_to_buf
Handle CloseComplete Postgres response in _parse_execute_to_buf

The `_parse_execute_to_buf`, which is used for JSON-based queries is
currently forgetting to handle the `CloseComplete` (3) response message
from Postgres despite engaging in prepared statement cleanup like its
cousin `_parse_execute` (which handles `CloseComplete` correctly).  This
leads to crashes when the prepared statement cache buffer overflows, and
is most prominent on HTTP ports, however it's also possible to trigger
this via a normal connection, because prepared statements also get
invalidated on schema version changes, as @vpetrovykh uncovered recently
in #1780.

Fixes: #1334
Fixes: #1314
Fixes: #1770

edgedb/edgedb,edgedb/edgedb,edgedb/edgedb