Add a per-request flag that restricts query execution
to the local node by filtering out all non-local replicas.
Standard consistency level (CL) rules still apply:
if the local node alone cannot satisfy the
requested CL, an exception is thrown.
This flag is required for Paxos state access, where
reads and writes must target only the local node.
As a side effect, this also enables the implementation
of scylladb/scylladb#16478, which proposes a CQL
extension to expose 'local mode' query execution to users.
Support for this flag in storage_proxy's read and write
code paths will be added in follow-up commits.
