Improve Multisite object caching flushing

[tillkruss]

Extracted from https://github.com/WordPress/performance/issues/36#issuecomment-999106773:

Consider supporting "per blog" flushing in multisite environments.

Object Cache Pro has a similar feature that allows flushing per blog and can even include global/shared groups, such as users. It also adds a "Flush" site action under Network Admin > Sites.

[eclarke1]

@tillkruss thanks so much for adding the 'Needs Dev' label, if it's ok I'll pop this and #105 into 'To Do' so they get flagged in the next weekly meeting as being available

[tillkruss]

Both this and #105 need a lot more discussion, there isn't enough information to get started on anything.

[eclarke1]

Sure thing @tillkruss in that case I'll remove the 'Needs Dev' label then otherwise this issue will show up in our links to the community as a 'request for development', when we share that link in the weekly meetings, thanks