add periodic gc.collect() callback to workers

[pfackeldey]

DO NOT MERGE

We should test and fine-tune (especially the frequency argument) this periodic callback.

This might be useful, because dask does call gc.collect() only during spilling or pausing, but that's likely too late.

[pfackeldey]

Usage:

import datetime
from dask.utils import parse_bytes

periodic_gc = PeriodicGC(
  freq=datetime.timedelta(milliseconds=1),
  thresh=parse_bytes("100 MB"),
)

client.register_plugin(periodic_gc)