Revive new client and fix relay-halt issue (#2161)

* Revert "Revert new client (#2153)"

This reverts commit 17cf7b59.

* add comment re cache to CachingClient (#2162)

* do not wait for reader when sending items (#2163)

* Use unsync cache + async-aware sync primitives (#2164)

* use unsync cache + async-aware sync primitives

* clippy

* trigger CI

* Revert "trigger CI"

This reverts commit b5c14054.