finagle-core: Move PanicMode out of LoadBalancerFactory and document …

…Panic Mode

Problem:
The `PanicMode` class is inside LoadBalancerFactory, which is hard to access.

We don't have docs in the Finagle clients section about panic mode.

Solution:
Move `PanicMode` class from
`com.twitter.finagle.loadbalancer.LoadBalancerFactory` to
`com.twitter.finagle.loadbalancer.PanicMode`.

Introduce the API `.withLoadBalancer.panicMode` to set the panic mode. Add docs
in the Finagle user guide client section.

JIRA Issues: CSL-10897

Differential Revision: https://phabricator.twitter.biz/D879034

kafka-streams/kafka-streams-queryable-thrift-client/src/main/scala/com/twitter/finagle/loadbalancer/ShardIdAwareRoundRobinBalancer.scala

@@ -3,7 +3,6 @@ package com.twitter.finagle.loadbalancer
 import com.twitter.finagle.Address.Inet
 import com.twitter.finagle._
 import com.twitter.finagle.context.Contexts
-import com.twitter.finagle.loadbalancer.LoadBalancerFactory.PanicMode
 import com.twitter.finagle.partitioning.zk.ZkMetadata
 import com.twitter.finagle.stats.StatsReceiver
 import com.twitter.finatra.streams.queryable.thrift.domain.RequestedShardIds