Limit log size for MTR

As defined in the specifications, the buildbot master can handle around 10k lines of logs per second. When multiple builds are running, MTR can produce more than this. The MTR step needs to be changed so that not all mysqld* logs are sent to the master process. Instead, the remaining logs will be transferred as txt files and a link should be shown in the build interface to point to the appropriate log file. In this way, the master process will not be overwhelmed by the huge amount of logs that it needs to process.