`seastar::current_backtrace()` can be quite heavey.
When we pass it to a log message in relatively detailed log_level
(debug/trace), we pay the price of `current_backtrace` every time,
but we rarely print the message.
Closes#13527
* github.com:scylladb/scylladb:
locator/topology: call seastar::current_backtrace only when log_level is enabled
schema_tables: call seastar::current_backtrace only when log_level is enabled