Prior to this commit, the SQLPanel would monkey patch the .cursor() and
.chunked_cursor() methods of each DatabaseWrapper connection in
.enable_instrumentation(), and then undo the monkey patch in
.disable_instrumentation().  However, for the same reasons as
a6b65a7, stop trying to undo the monkey
patching in .disable_instrumentation() and simply use a .djdt_logger
attribute on the DatabaseWrapper to determine if the cursors should be
wrapped.

Several tests (such as SQLPanelTestCase.test_cursor_wrapper_singleton)
are written to ensure that only a single cursor wrapper is instantiated
during the test.  However, this fails when using Django's psycopg3
backend, since the .last_executed_query() call in
NormalCursorWrapper._record() ends up creating an additional cursor (via
[1]).  To avoid this wrapping this additional cursor, set the
DatabaseWrapper's ._djdt_logger attribute to None before calling
.last_executed_query() and restore it when finished.  This will cause
the monkey-patched DatabaseWrapper .cursor() and .chunked_cursor()
methods to return the original cursor without wrapping during that call.

[1] https://github.com/django/django/blob/4.2.1/django/db/backends/postgresql/psycopg_any.py#L21

This switches the Debug Toolbar cursor wrappers to inherit from the
Django class django.db.backends.utils.CursorWrapper. This reduces some
of the code we need.