Fix for index block appearing as a flushed index block, now explicit check has on disk segment

[robskillington]

What this PR does / why we need it:

This changes the logic to detect whether a block is flushed or not (and consequently can GC in-memory series from index). Previously was checking that the block had been sealed (had at least started transition to on disk segment) and that no in-memory series were held by block any longer. This is a poor check since in-memory series are held by the active block not the block for that block start, so would return true even though the block had not been flushed.

Now the check is much more robust since it checks the result of the call to block.Tick(...) which explicitly returns whether it has a bootstrapped segment (i.e. a segment which is an on disk segment) held as a reference or not, which is the root for the source of truth on whether the in-memory series for that block start can be GC'd or not.

Special notes for your reviewer:

Does this PR introduce a user-facing and/or backwards incompatible change?:

NONE

Does this PR require updating code package or user-facing documentation?:

NONE

[codecov]

Codecov Report

Merging #3622 (861d4b0) into r/index-active-block (ed89e86) will decrease coverage by 0.0%.
The diff coverage is 90.0%.

@@                  Coverage Diff                   @@
##           r/index-active-block   #3622     +/-   ##
======================================================
- Coverage                  56.2%   56.1%   -0.1%     
======================================================
  Files                       551     551             
  Lines                     62170   62178      +8     
======================================================
- Hits                      34974   34921     -53     
- Misses                    24055   24128     +73     
+ Partials                   3141    3129     -12     

Flag	Coverage Δ
aggregator	57.1% <ø> (ø)
cluster	∅ <ø> (∅)
collector	58.4% <ø> (ø)
dbnode	60.6% <90.0%> (-0.2%)	⬇️
m3em	46.4% <ø> (ø)
metrics	19.8% <ø> (ø)
msg	74.4% <ø> (+0.1%)	⬆️

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