Fixes long wait filter keyspace

[rafael]

Description

• The following PR fixes an issue with KeyspacesToWatch functionality. When this flag is provided, some keyspaces are removed. However, vtgates still try to wait for available tablet types in them. This PR makes sure that filter is respected.
• I also added some tests around WaitForTablets for existent and new functionality.

[deepthi]

do you know whether this also needs to be fixed on the new gateway (TabletGateway)?

[deepthi]

The implementation feels a bit fragile. We are mutating the arguments passed to an exported function. Even though existing uses of that function are such that this won't cause problems, given that it is an exported function, I feel that it is not a good idea.
How about creating a new slice that consists of only the relevant keyspaces and returning that instead of operating directly on targets?

[rafael]

The implementation feels a bit fragile. We are mutating the arguments passed to an exported function. Even though existing uses of that function are such that this won't cause problems, given that it is an exported function, I feel that it is not a good idea.
How about creating a new slice that consists of only the relevant keyspaces and returning that instead of operating directly on targets?

I think this is a good idea. I was on the fence because the existent code already mutates this same slice. I updated the code with your suggestion. We should probably do something similar here

[deepthi]

LGTM