- Related: #6359

I tested that code and this cant happen. We need to iterate keys and
then ask 'del'. The previous method raised: RuntimeError: dictionary
changed size during iteration

When handling this exception Python got lost because the import was
incorrect. Queue.Empty comes from Queue, not from multiprocessing.Queue

Instead of opening one TCP connection for each HTTP request, we want to reuse
connections. Also, we need to be able to verify SSL certificates. This commit
implements both features in the twisted http client sync.

The whole idea of the encrypter/decrypter pool is to be able to use multiple
cores to allow parallel encryption/decryption. Previous to this commit, the
encryptor/decryptor pools could be configured to not use workers and instead
do encryption/decryption inline. That was meant for testing purposes and
defeated the purpose of the pools.

This commit removes the possibility of inline encrypting/decrypting when using
the pools. It also refactors the enc/dec pool code so any failures while using
the pool are correctly grabbed and raised to the top of the sync deferred
chain.

When we initialized the async decrypter pool in the target's init method we
needed a proxy to ensure we could update the insert doc callback with the
correct method later on. Now we initialize the decrypter only when we need it,
so we don't need this proxy anymore. This commit removes the unneeded proxy.

We have to make sure any failures in asynchronous decryption code is grabbed
and properly transmitted up the deferred chain so it can be logged. This
commit adds errbacks in the decryption pool that grab any failure and a
check on the http target the failure if that is the case.