Reduce rebuilds and build time by removing unnecessary includes. Along the way,
improve header sanity.
Ref #1.
Test: dev-headers, unit(dev).
Closes#8524
* github.com:scylladb/scylla:
treewide: remove inclusions of storage_proxy.hh from headers
storage_proxy: unnest coordinator_query_result
treewide: make headers self-sufficient
utils: intrusive_btree: add missing #pragma once