[dbnode] Avoid data race in xcontext

[linasm]

What this PR does / why we need it: There is a data race on checkedAndNotSampled field in xcontext which happens when the context is being used concurrently. I believe there is no production impact, but this prevents the running of multiple tests with -race.

An alternative would be to agree that xcontext is not meant to be thread safe and avoid using it across goroutines (see https://github.com/m3db/m3/pull/3261) but from the usage of lockking elsewhere it looks like the original intention was for it to be thread safe.

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[bot]]

Codecov Report

Merging #3283 (ed45a1d) into master (da44ea0) will decrease coverage by 0.1%. Report is 680 commits behind head on master. The diff coverage is 94.1%.

Additional details and impacted files

@@            Coverage Diff            @@
##           master    #3283     +/-   ##
=========================================
- Coverage    72.4%    72.4%   -0.1%     
=========================================
  Files        1099     1099             
  Lines      101504   101518     +14     
=========================================
+ Hits        73579    73588      +9     
- Misses      22849    22856      +7     
+ Partials     5076     5074      -2     

Flag	Coverage Δ
aggregator	76.5% <ø> (+<0.1%)	:arrow_up:
cluster	84.9% <ø> (ø)
collector	84.3% <ø> (ø)
dbnode	78.8% <0.0%> (-0.1%)	:arrow_down:
m3em	74.4% <ø> (ø)
m3ninx	73.5% <ø> (ø)
metrics	19.9% <ø> (ø)
msg	74.0% <ø> (-0.2%)	:arrow_down:
query	67.3% <ø> (ø)
x	80.5% <100.0%> (+<0.1%)	:arrow_up:

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