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)
```
Micha Gorelick
a9a15966e2