client: use appropriate ExecutionContext for initial sync / regular import (#6180)

* client: use appropriate ExecutionContext for sync/import

* client: remove dead code

* client: ExecutionContext: distinguish between own and foreign imports

* client: fix cli parameter doc

* Revert "client: ExecutionContext: distinguish between own and foreign imports"

This reverts commit 0fac1152.

* primitives: add docs for ExecutionContext

* cli: execution strategy docs

* cli: use different execution context for importing block on validator

* cli: remove defaults from execution context flags