Bound connection attempt concurrency in ConnectionManager

[ReubenBond]

This PR changes how ConnectionManager is implemented, especially in cases where there are multiple tasks waiting on a connection attempt.

• Use a single lock for all connection management
• Avoid the ~async spinlock and instead have waiting tasks wait on the pending connection attempt directly
• Initiate connection task under lock, yielding immediately
• When there are insufficient connections for an endpoint, initiate additional connections in the background and return synchronously if there is at least one valid connection already

[sergeybykov]

Should we .Ignore() this task?

[ReubenBond]

I'll add that or make sure the task cannot throw