clear responsibilities

[tudor-malene]

Why this change is needed

in preparation for adding the Session keys functionality to the Ten gateway, we need to structure the business logic a bit better

What changes were made as part of this PR

• move the rpc connection pool to a separate type
• only use a single type of "User" object in the codebase.
• create a separate cache for the users
• use the StorageWithCache

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[zkokelj]

Few minor things:

tools/walletextension/storage/storage_test.go is broken
(it should be a quick fix.. adding logger to new storage and renaming user.PrivateKey to user.UserKey)

I was thinking about caching users in the gateway.
If we want to run multiple gateway backends this might be a bit of a problem, since each has separate cache and for example adding a user won't invalidate cache on another gateway instance and that instance will continue to think that this new address was not authenticated by this user.
Just something to keep in mind.

Otherwise LGTM