New balancer and resolver APIs

[menghanl]

This is the issue to track progress on the new balancer and resolver APIs.
Please see gRFC L9 for details: grpc/proposal#30 (discussion link)

• Add new interfaces and wrapper for the old balancer Add new Resolver and Balancer APIs (gRFC L9) #1408
• New implementation for balancers, including pickfirst, roundrobin and grpclb
  • pickfirst New implementation of roundrobin and pickfirst #1506
  • roundrobin New implementation of roundrobin and pickfirst #1506
  • grpclb New grpclb implementation #1558
• Support switching balancers
  • based on service config switch balancer based on service config info #1670
  • based on address type (use grpclb if resolved addresses contain grpclb) Switch balancer to grpclb when at least one address is grpclb address #1692

The issue with failfast(as mentioned in #1432) will also be fixed.

[trusch]

Just for reference,I tested this for my usecase (see #1694) and I noticed two things:

• The query interval of the DNS resolver is not configurable (30min)
• Dead connections are also retried 30mins before getting removed