BSON: Autogenerate bson_config.h.in.
Build: Restored some lost casts.
Protocols.HTTP.Query: Support async keep-alive.

If a Protocols.HTTP.Query instance is used with a keep-alive connection,
async_request fails if the remote end closed the connection.

Fixes [bug 7143].
Removed #if 0 code.
Basic constrants cA is declared false by default, so en empty sequence is legal (and more space efficient).
Make some debug less invasive.
Unused defines.