Don't keep binary log index file locked during show binary logs

While doing show binary logs, the index_file mutex is kept locked during the reading of the index file and while calculating the length of each binary file.

On some systems with 10,000+ binlogs, show binary logs could block log rotation for more than 10 seconds. When this happens, the server is locked for commits during the binary log rotation.

The fix is to not keep the mutex locked while calculating the file sizes.