TB 3455: Add DomainIsolator, for isolating circuit by domain.

Add an XPCOM component that registers a ProtocolProxyChannelFilter
which sets the username/password for each web request according to
url bar domain.

Bug 9442: Add New Circuit button

Bug 13766: Set a 10 minute circuit dirty timeout for the catch-all circ.

Bug 19206: Include a 128 bit random tag as part of the domain isolator nonce.

Bug 19206: Clear out the domain isolator state on `New Identity`.

Bug 21201.2: Isolate by firstPartyDomain from OriginAttributes

Bug 21745: Fix handling of catch-all circuit

Bug 41741: Refactor the domain isolator and new circuit