Persist attribute writes in the database

[puddly]

Fixes #958

[coveralls]

Coverage remained the same at 99.738% when pulling a25ba0d on puddly:puddly/cache-attr-writes into ab52cad on zigpy:dev.

[codecov-commenter]

Codecov Report

Merging #962 (a25ba0d) into dev (ab52cad) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##              dev     #962   +/-   ##
=======================================
  Coverage   99.74%   99.74%           
=======================================
  Files          45       45           
  Lines        6689     6689           
=======================================
  Hits         6672     6672           
  Misses         17       17           

Impacted Files	Coverage Δ
zigpy/zcl/__init__.py	99.76% <100.00%> (ø)

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