bigtable/bttest: Support multiple versions of a cell.
bigtable/bttest: Support multiple versions of a cell.

Also implement cells_per_column_limit_filter support.

Change-Id: I86afe90abe6f63d1f0694bca258a4959edfbed98

hsinhoyeh/gcloud-golang,ptomasroos/gcloud-golang,Maksadbek/gcloud-golang,roth1002/gcloud-golang,googleapis/google-cloud-go,michael-grosshans/google-cloud-go,michael-grosshans/google-cloud-go,hsinhoyeh/gcloud-golang,roth1002/gcloud-golang,googleapis/google-cloud-go,blowmage/gcloud-golang,blowmage/gcloud-golang,ptomasroos/gcloud-golang,Maksadbek/gcloud-golang,googleapis/google-cloud-go,googleapis/google-cloud-go