Add a skip condition to all tests slower than 100ms.
This change was made using `gotestsum tool slowest` with data from the
last 3 CI runs of master.
See https://github.com/gotestyourself/gotestsum#finding-and-skipping-slow-tests
With this change:
```
$ time go test -count=1 -short ./agent
ok github.com/hashicorp/consul/agent 0.743s
real 0m4.791s
$ time go test -count=1 -short ./agent/consul
ok github.com/hashicorp/consul/agent/consul 4.229s
real 0m8.769s
```
Highlights:
- add new endpoint to query for intentions by exact match
- using this endpoint from the CLI instead of the dump+filter approach
- enforcing that OSS can only read/write intentions with a SourceNS or
DestinationNS field of "default".
- preexisting OSS intentions with now-invalid namespace fields will
delete those intentions on initial election or for wildcard namespaces
an attempt will be made to downgrade them to "default" unless one
exists.
- also allow the '-namespace' CLI arg on all of the intention subcommands
- update lots of docs
