Small optimization in appendValuesInstant.

made valueAndMeta struct package private.

OrderedFlush() now sorts the results instead popping them one by one.
FloatHeap won't be created if it's not used.

FloatHeap now supports adding NaN values.
Uncommented topk, bottomk and sort compatibility tests (now they pass).

equalsPairs now checks individual struct elements instead of their printed values.

Used append in place when indices are not needed.
Documented and extracted some comparison functions for better readability and clearness.

* master:
  [query] Improve precision for variance and stddev of equal values (#2799)
  Support dynamic namespace resolution for embedded coordinators (#2815)
  [dbnode] Add index regexp DFA compilation cache to avoid allocating DFAs for same expressions (#2814)
  [dbnode] Set default cache on retrieve to false, prepare testing with cache none (#2813)
  [tools] Output annotations as base64 (#2743)
  Add Reset Transformation (#2794)
  [large-tiles] Fix for a reverse index when querying downsampled namespace (#2808)
  [dbnode] Evict info files cache before index bootstrap in peers bootstrapper (#2802)
  [dbnode] Bump default filesystem persist rate limit (#2806)
  [coordinator] Add metrics and configurability of downsample and ingest writer worker pool (#2797)
  [dbnode] Fix concurrency granularity of seekerManager.UpdateOpenLease (#2790)
  [dbnode] Use correct import path for atomic library (#2801)
  Regenerate carbon automapping rules on namespaces changes (#2793)
  Enforce matching retention and index block size (#2783)
  [query] Add ClusterNamespacesWatcher and use to generate downsample automapper rules (#2782)
  [query][dbnode] Wire up etcd-backed Clusters implementation in coordinator (#2758)
  [query] Fix quantile() argument not being passed through (#2780)
  [query] Add "median" aggregation to Graphite aggregate() function (#2774)
  [r2] Ensure KeepOriginal is propagated when adding/reviving rules (#2796)
  [dbnode] Update default db read limits  (#2784)

* master:
  [query] count_values label naming fixes (#2805)
  [dbnode] Read only namespaces (#2803)
  Remove old code: m3nsch (#2822)
  Update issue templates
  Update issue templates
  [proto] Allow zero-alloc reuse of AggregatedMetric protobuf payloads (#2823)
  Remove old code: aggregator/handler/trafficcontrol (#2821)
  Remove old code: aggregation/quantile/tdigest (#2820)
  AddToReset config to change Add transforms to Reset transforms (#2817)
  [metrics/rules] Add test to assert that mixed-mode keepOriginal uses keepOriginal=true (#2819)
  [dbnode] Update return unfulfilled for corrupt commit log files default (#2807)
  [dbnode] Move eviction logic up before ns loop (#2812)
  [query] Add additional parser tests (#2811)
  [config] Remove deprecated configuration fields (#2771)

* master:
  [read_index_segments] Always validate index segment checksums before reading/validating contents (#2835)
  [query] Return additional warnings in /query{,_range} endpoints (#2836)
  Add a check for seriesIter.Err (#2834)
  [tools] Add concurrent read_index_segments validation option (#2833)
  [query] Add non-ready namespaces to Clusters interface and use in /namespace/ready endpoint (#2828)
  [query] Tests for when function argument is an expression (#2809)
  [large-tiles] Additional metrics added (#2720)
  [query] Refactor groupByNodes and implement `nodes` arg in asPercent (#2816)
  [read_index_segments] Add log that outlines which segment being processed (#2825)
  [aggregator] Handle follower canLead() for not yet flushed windows (#2818)