[rpc module]: server-side close subscription (#355)

* [rpc module]: server-side close subscription

Add functionality that closes the subscription after the sink has been dropped.

* [integration tests]: add timeout on futures

* remove global subscriber mutex

* fix nit

* [client types]: fix #349

Subscription::next() propogate error when parsing the response fails

* [client types]: fix #349

Subscription::next() propogate error when parsing the response fails

* unify subscription and notification

* rename again

* send notif response when subscription is dropped

* stray debug stuff

* Update utils/src/server/rpc_module.rs

Co-authored-by: David <[email protected]>

* Revert "[client types]: fix #349"

This reverts commit c4fefade719ebd8c964a62d0eb16f89db3346ccd.

* Revert "[integration tests]: add timeout on futures"

This reverts commit 21dfb99649aaaa8c847a693b4510e0202498fa2c.

* show that actual edge-case

* fix nit

* Update types/src/traits.rs

* fix bad merge

* ugly; but works

* complete solution

* get rid of Option

* Update tests/tests/integration_tests.rs

* Update utils/src/server/rpc_module.rs

* Update utils/src/server/rpc_module.rs

Co-authored-by: David <[email protected]>

* grumbles: fix faulty early return

* remove weird abstraction KeepAlive

* fix nits

* revert test timeouts

* address grumbles

* fix build

Co-authored-by: David <[email protected]>