add mt-index-deleter tool

[Dieterbe]

deletes mkeys provided on stdin from bigtable index.

seems to work really fast, btw. first run was already super fast so i didn't even try to tweak the settings

/ # cat rowkeys.txt  | ./mt-index-deleter --gcp-project ... --bigtable-instance ...  --bigtable-table ... --batch 10000
2020-11-09 21:06:46.998 [INFO] bt-ensure-table-exists: table "..." exists.
INFO[0000] workerstats: 0 deleted in 50.158µs (0.000000 deletes/s)  file=worker.go line=21
INFO[0060] workerstats: 9350000 deleted in 1m0.000188811s (155832.842951 deletes/s)  file=worker.go line=21
INFO[0120] workerstats: 17470000 deleted in 2m0.000373848s (145582.879784 deletes/s)  file=worker.go line=21

[stale]

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