xds: make balancer group restartable (#2999)

This is a preparing change to support priority failover. It adds start() and close() to balancer group, so we can have a balancer group that's not in use, but has all the data and is ready to be started (think about a lower priority when the higher priority is in use).

A balancer group is split into two parts: static and dynamic:

static: the data from EDS, and gets updated even if balancer group is closed
balancer IDs and builders, addresses for each balancer
dynamic: the sub-balancers
These are only created when the balancer group is started. They are closed when the balancer group is closed.
And only when the balancer group is started, the sub-balancers will get address updates.