Add `explain` flag support to the reroute API

By specifying the `explain` flag, an explanation for the reason a
command can or cannot be executed is returned. No allocation commands
are actually performed.

Returns a response similar to:

{
  "state": {...cluster state...},
  "acknowledged": true,
  "explanations" : [ {
    "command" : "cancel",
      "parameters" : {
        "index" : "decide",
        "shard" : 0,
        "node" : "IvpoKRdtRiGrQ_WKtt4_4w",
        "allow_primary" : false
      },
      "decisions" : [ {
        "decider" : "cancel_allocation_command",
        "decision" : "YES",
        "explanation" : "..."
        } ]
     }, {
      "command" : "move",
      "parameters" : {
        "index" : "decide",
        "shard" : 0,
        "from_node" : "IvpoKRdtRiGrQ_WKtt4_4w",
        "to_node" : "IvpoKRdtRiGrQ_WKtt4_4w"
       },
       "decisions" : [ {
         "decider" : "same_shard",
         "decision" : "NO",
         "explanation" : "shard cannot be allocated on same node [IvpoKRdtRiGrQ_WKtt4_4w] it already exists on"
       },
       etc
       ]
  }]
}

also removes AllocationExplanation from cluster state

Closes #2483
Closes #5169
Remove trace logging from testBreakerWithRandomExceptions()
Improve error reporting on breaker stats assertion
Update Eclipse configuration to Java 1.7.
Add assertion to not forget to replace reflection with the new FilterAtomicReader.unwrap.
Move ensureEstimatedStats() after index wiping to ensure everything is cleaned up once the index is removed
Add separate thread pool for global repository operations

Fixes #5240