refactor RPC context to provide RPC specific storages (e.g. subscriptions)

[xlc]

          One downside about global variables is that for some use cases, we need to handle chopsticks instances running at a same time (e.g. unit tests) so need to ensure they don't interfere with each other. 

In this particular case, it is fine as everything is indexed by a unique random ID. The only drawback I can see is potential memory leak (i.e. destroy a chopsticks instance will not clear entry here). But that's not a very big deal here. So this is ok for now. Maybe later refactor this into context somehow.

Originally posted by @xlc in https://github.com/AcalaNetwork/chopsticks/pull/838#discussion_r1802044117