Split off do_execute() into a fast path and slow(ish) path, and
coroutinize the latter.
perf-simple-query shows no change in performance (which is
unsurprising since it picks the fast path which is essentially unchanged).
Closes#14246
* github.com:scylladb/scylladb:
cql3: select_statement: reindent execute_without_checking_exception_message_aggregate_or_paged()
cql3: select_statement: coroutinize execute_without_checking_exception_message_aggregate_or_paged()
cql3: select_statement: split do_execute into fast-path and slow/slower paths
cql3: select_statement: disambiguate execute() overloads
9e9636ef15