Fix DeleteCellInfo(force=true) with downed local topo

[ajm188]

Signed-off-by: Andrew Mason amason@slack-corp.com

Description

Implements the fix proposed in #8220. When GetSrvKeyspaceNames fails (for reasons other than NoNode) and the context expired, we assume it's because the local cell is shutdown, spin up a background context, and delete it anyway.

Manual testing

❯ vtctlclient -server "localhost:15999" AddCellInfo -server_address "localhost:99999" -root "/vitess/zone2" zone2
❯ vtctlclient -server ":15999" GetCellInfoNames
zone1
zone2
❯ vtctlclient -server "localhost:15999" DeleteCellInfo zone2
DeleteCellInfo Error: rpc error: code = Unknown desc = can't list SrvKeyspace entries in the cell; use -force flag to continue anyway (e.g. if cell-local topo was already permanently shut down): failed to create topo connection to localhost:99999, /vitess/zone2: context deadline exceeded
E1025 11:20:39.650251   35350 main.go:76] remote error: rpc error: code = Unknown desc = can't list SrvKeyspace entries in the cell; use -force flag to continue anyway (e.g. if cell-local topo was already permanently shut down): failed to create topo connection to localhost:99999, /vitess/zone2: context deadline exceeded
❯ vtctlclient -server "localhost:15999" DeleteCellInfo -force zone2
❯ vtctlclient -server ":15999" GetCellInfoNames
zone1

Related Issue(s)

Fixes #8220

Checklist

• Should this PR be backported? no
• Tests were added or are not required not possible with memorytopo
• Documentation was added or is not required

Deployment Notes

[deepthi]

Looks fine.
Ideally we'd have a unit test for this. Do you think you could make one?

[ajm188]

Can't block my own PR, so I'm going to label this do-not-merge and work on that unit test!