performance fix: pruning effect on latency, go 1.11, etc

• when pruning index, use more granular locking (prune individual metrics separately rather then all at once). this can significantly reduce request latencies, as previously, some render requests could be blocked a long time by long running index prunes (which was especially worse with high series churn). now there is practically no latency overhead (though prunes now run longer but that's not a problem) #1062
• emit the current MT version as a metric #1041
• k8s: allow passing GO* variables as MT_GO* #1044
• better docs for running MT via docker #1040, #1047
• make fragile duration integer config values parseable as string durations #1017
• go 1.11 #1045
• no need for $(go list ./... | grep -v /vendor/) #1050