Feature Request: reduce topo calls in tabletgateway's WaitForTablets(...)

[timvaillancourt]

Feature Description

This issue discusses reducing topo calls in tabletgateway's WaitForTablets(...) when --keyspaces_to_watch is provided

Today, the .WaitForTablets(...) func calls srvtopo.FindAllTargets(...) (from go/vt/srvtopo) that has no option to filter by keyspace, so it does fetches for every available keyspace. Later the targets that don't match --keyspaces_to_watch are nil-ed out in go/vt/discovery

The improvement proposed is to not-fetch keyspaces we don't need when --keyspaces_to_watch. This would happen earlier by passing the expected keyspaces to srvtopo.FindAllTargets(...) and supporting a fallback when no --keyspaces_to_watch is defined

Use Case(s)

Deployments of vtgate with --keyspaces_to_watch defined