CQL transport sever error handling fixes and improvements:
* log failed requests with `DEBUG` level for easier debugging;
* in case of unhandled errors, deliver them to the client as `SERVER_ERROR`'s
* fix for `protocol_error`'s in case of shedded big requests;
* explicit tests have been written for the error handling problems above.
Closes#11949
* github.com:scylladb/scylladb:
transport server: fix "request size too large" handling
transport server: log failed requests with debug level
transport server: fix unexpected server errors handling
transport server: log client errors with debug level