Consider keying the HTTP connection pool based on the supplied credential

Currently, the connection pool is keyed off of the type of connection / host / port / SNI / Proxy URL.
Credentials are resolved after the fact (once we already have a connection), so they don't impact how we partition connections.
With NTLM, this can have unintended consequences, so we should look at how feasible it is to make this work.

We made a previous change to special-case the `CredentialCache.DefaultCredentials` in #303 to include the identity as part of the partitioning key, but this doesn't work for any sort of custom credential type.

Start by reading the connection-pool partitioning behavior described here and the prior change in #303, including how CredentialCache.DefaultCredentials is handled. Investigate how custom credentials and NTLM affect connection reuse, then define a feasible credential-based partitioning approach and tests that demonstrate correct separation.

Written by the indexing model from the issue text.