[vdsql-] ensure each thread in vdsql tracks its single connection

[anjakefala]

Uses threading.local to keep track of a thread's connection.

Otherwise, you occassionally get this error when opening an SQLite table with vdsql:

Traceback (most recent call last):
  File "/home/anja/anaconda3/envs/visidata/lib/python3.11/threading.py", line 995, in _bootstrap
    self._bootstrap_inner()
  File "/home/anja/anaconda3/envs/visidata/lib/python3.11/threading.py", line 1038, in _bootstrap_inner
    self.run()
  File "/home/anja/anaconda3/envs/visidata/lib/python3.11/threading.py", line 975, in run
    self._target(*self._args, **self._kwargs)
  File "/home/anja/git/visidata/visidata/threads.py", line 209, in _with_active_cmd
    _toplevelTryFunc(func, *args, **kwargs)
  File "/home/anja/git/visidata/visidata/threads.py", line 227, in _toplevelTryFunc
    t.status = func(*args, **kwargs)
  File "/home/anja/git/visidata/visidata/sheets.py", line 281, in reload
    self.loader()
  File "/home/anja/git/visidata/visidata/sheets.py", line 307, in loader
    for i, r in enumerate(self.iterload()):
  File "/home/anja/git/visidata/visidata/apps/vdsql/_ibis.py", line 357, in iterload
    self.query = self.baseQuery(con)
  File "/home/anja/git/visidata/visidata/apps/vdsql/_ibis.py", line 345, in baseQuery
    tbl = con.table(self.table_name)
  File "/home/anja/anaconda3/envs/visidata/lib/python3.11/site-packages/ibis/backends/sql/__init__.py", line 100, in table
    table_schema = self.get_schema(name, catalog=catalog, database=database)
  File "/home/anja/anaconda3/envs/visidata/lib/python3.11/site-packages/ibis/backends/sqlite/__init__.py", line 298, in get_schema
    with self.begin() as cur:
  File "/home/anja/anaconda3/envs/visidata/lib/python3.11/contextlib.py", line 137, in __enter__
    return next(self.gen)
  File "/home/anja/anaconda3/envs/visidata/lib/python3.11/site-packages/ibis/backends/sqlite/__init__.py", line 147, in begin
    cur = self.con.cursor()
sqlite3.ProgrammingError: SQLite objects created in a thread can only be used in that same thread. The object was created in thread id 139669398107904 and this is thread id 139668276848384.