This is necessary to support newest TSDB.

Newest optimization (http://github.com/prometheus/tsdb/pull/486) makes tsdb.PostingForMatcher impossible to use.

This also hopefully reduce amount of code to understand as we don't need to necessarily fit into
index.Postings even though we don't need streaming for now. This should make the code less complex and more readable
(in comparision to previous `lazyPostings` implementation.

Signed-off-by: Bartek Plotka <bwplotka@gmail.com>

This PR does not add anything, however by upgrading Prometheus from 2.3.2 to master tip and same for TSDB it may affects few things.
Bigger packages we use and changes I found manually:
* prometheus/prometheus/discovery/file
  * [ENHANCEMENT] Discovery: Improve performance of previously slow updates of changes of targets. #4526 ?? cc @ivan-valkov
  * [BUGFIX] Wait for service discovery to stop before exiting #4508 ??
* prometheus/prometheus/promql:
  * [BUGFIX] PromQL: Fix a goroutine leak in the lexer/parser. #4858
  * [BUGFIX] Change max/min over_time to handle NaNs properly. #438
  * [BUGFIX] Check label name for `count_values` PromQL function. #4585
  * [BUGFIX] Ensure that vectors and matrices do not contain identical label-sets. #4589
  * [ENHANCEMENT] Optimize PromQL aggregations #4248
  * [BUGFIX] Only add LookbackDelta to vector selectors #4399
  * [BUGFIX] Reduce floating point errors in stddev and related functions #4533

* prometheus/prometheus/rules:
  * New metrics exposed! (prometheus evaluation!)
  * [ENHANCEMENT] Rules: Error out at load time for invalid templates, rather than at evaluation time. #4537

* prometheus/tsdb/index: Index reader optimizations.

There are things/fixes we may reuse in next PRs (TODO create gh issues for those):
* api changes (warnings support on Prometheus UI and Query API)
* UI fixes:
  * [ENHANCEMENT] Web UI: Support console queries at specific times. #4764
  * [ENHANCEMENT] Web UI: Avoid browser spell-checking in expression field. #472
* Use notifier package once prometheus/prometheus#5025 is merged.
* Ruler UI fixes:
 * [ENHANCEMENT] Show rule evaluation errors in UI #4457

Follopw up issues we can fix in further PRs:
* QueryAPI has now api/v1/labels that Thanos does not yet support. Created issue with details: #702
* #703

Signed-off-by: Bartek Plotka <bwplotka@gmail.com>

# Conflicts:
#	pkg/store/bucket.go
#	pkg/store/bucket_e2e_test.go