grpc-js: Simplify pick_first behavior

[murgatroid99]

This includes a few related changes to the pick_first LB policy to simplify its behavior, in order to reduce the chances of reaching a bad state, and to make it easier to reason about:

• In pickSubchannel, avoid assuming that the subchannel to be picked has any state related to this LB policy instance; in particular, don't assume that it has the state change listener registered, and don't have special case behavior if it's the same subchannel as the current pick. Instead, unconditionally reset the LB policy state and set up the relevant subchannel state from scratch.
• In resetSubchannelList, remove the special case when handling the current pick.
• In connectToAddressList, don't take any additional actions when immediately picking one of the new subchannels.
• In removeCurrentPick, call unref last to completely avoid calling the connectivity state listener.
• In updateAddressList, switch requestedResolutionSinceLastUpdate to false immediately instead of assuming that will be done elsewhere.

[murgatroid99]

The last commit removes triedAllSubchannels and requestedResolutionSinceLastUpdate.

triedAllSubchannels just conditionally skips startNextSubchannelConnecting, but calling that an extra time shouldn't be a problem. It might just cause some extra connection attempts.

requestedResolutionSinceLastUpdate was used to request resolution exactly once each time every subchannel fails a connection attempt. Now instead the hasReportedTransientFailure is just reset each time it becomes true for every subchannel.