indexer: use multiple processes, e.g. process per indexer

[karlicoss]

At the moment only auto provider has some ad-hoc indexing

promnesia/src/promnesia/sources/auto.py

Lines 211 to 218 in 127cf75

	cores = use_cores()
	if cores is None: # do not use cores
	pool = nullcontext()
	mapper = map # dummy pool
	else:
	workers = None if cores == 0 else cores
	pool = Pool(workers) # type: ignore
	mapper = pool.map # type: ignore

Need a flag/optional config option to allow specifying number of cores to run on + make sure that single core isn't spawning a separate process (otherwise breaks debugging). By default should be single threaded to avoid potential issues and simplify initial troubleshooting

Would require a change somewhere around here

promnesia/src/promnesia/__main__.py

Lines 37 to 38 in 127cf75

	def iter_all_visits() -> Iterator[Res[DbVisit]]:
	for idx in indexers: