Fixed racing in coordinator, when assignment is performed before metadata update

[tvoinarovskyi]

Fixes #118

[codecov-io]

Codecov Report

Merging #120 into master will increase coverage by 1%.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master     #120   +/-   ##
=======================================
+ Coverage   95.61%   96.61%   +1%     
=======================================
  Files          10       10           
  Lines        1800     1801    +1     
=======================================
+ Hits         1721     1740   +19     
+ Misses         79       61   -18

Impacted Files	Coverage Δ
aiokafka/client.py	98.3% <100%> (+0.84%)	✅
aiokafka/group_coordinator.py	94.27% <100%> (+0.01%)	✅
aiokafka/fetcher.py	93.96% <ø> (+0.52%)	✅
aiokafka/consumer.py	98.52% <ø> (+6.89%)	✅

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 4de6d52...893f62e. Read the comment docs.