This patch:
1. Changes `SimpleChannel` to buffer messages to the namespace that
hasn't been registered yet. This allows us to create `SimpleChannel`
per target on the browser side right away.
2. Removes multisession support. Now there's only one `PageAgent` in the
content process, which talks to a single `PageHandler` on the browser
side. Both ends can be created as-soon-as-needed; thanks to
`SimpleChannel` bufferring, no messages will be lost and all messages
will be delivered in proper order. (This is currently the reason why
build 1178 flakes on windows).
3. Straightens up the target reporting. Targets are reported as soon
as they appear on the browser side.
**NOTE:** this doesn't yet remove sessions from protocol.
References #3995
