Add optional callback to ChunkedInsert

[mynameisfiber]

This commit adds an optional callback argument to the ChunkedInsert object. This callback is a callable object which gets called before the chunked insert happens. This is useful for clearing any local caches that may be in place to deal with the eventual consistency resulting from the delayed nature of the chunked inserts.

For example,

cache = set()
chunked_table = ChunkedInsert(table, callback=lambda queue: cache.clear())
while True:
    data = get_data_id()
    key = data['key']
    if key in cache or table.find_one(key=key)
        continue
    cache.add(key)
    chunked_table.insert(data)

[pudo]

I'm a bit worried this will lead to some pretty wonky control flows, but let's give it a shot :)