Use ConnectionLease to return connection

[fabianfett]

This adds some functionality prototyped in #551. Passing a lease to the user, allows us to capture the pool, the connection came from. This ensures, we don't need to look for the pool the connection is from, when we release it.

[fabianfett]

API break is expected:

3 breaking changes detected in _ConnectionPoolModule:
  💔 API breakage: func ConnectionRequestProtocol.complete(with:) has parameter 0 type change from Swift.Result<Self.Connection, _ConnectionPoolModule.ConnectionPoolError> to Swift.Result<_ConnectionPoolModule.ConnectionLease<Self.Connection>, _ConnectionPoolModule.ConnectionPoolError>
  💔 API breakage: func ConnectionPool.leaseConnection() has return type change from Connection to _ConnectionPoolModule.ConnectionLease<Connection>
  💔 API breakage: func ConnectionRequest.complete(with:) has parameter 0 type change from Swift.Result<Connection, _ConnectionPoolModule.ConnectionPoolError> to Swift.Result<_ConnectionPoolModule.ConnectionLease<Connection>, _ConnectionPoolModule.ConnectionPoolError>