Invert the service discovery (#268)

* Draft: very messy and doesn't compile

* Simplifyy

* less log, plz

* simplify more

* Simplified by a lot - much simpler

now pick fewer addresses

* fixy

* Account for infinite

* copyright nonsense

* clean up debug logging

* round_robin works!

* use rw mutex to serialize creation

* rework the filtering to make everything parameterized and more explicit

Change all the config so that instead of hard coded constants we set the
various connection attributes, json field names, etc using command line flags.

Then make the pool type and affinity arguments more explicit and less generic.

* no longer needed

* update comments

* only pass through the URL params we need

* affinity is actually optional

---------

Co-authored-by: Michael Demmer <[email protected]>

go/vt/vtgate/vtgateconn/vtgateconn.go

@@ -1,5 +1,5 @@
 /*
-Copyright 2019 The Vitess Authors.
+Copyright 2024 The Vitess Authors.
 
 Licensed under the Apache License, Version 2.0 (the "License");
 you may not use this file except in compliance with the License.