fix: SSR memory leak

[Pindar]

by moving subscribeDataChanging calls into beforeMount according to
https://ssr.vuejs.org/guide/universal.html#component-lifecycle-hooks

However, because the destroy hooks will not be called during SSR, the timers will stay around forever. To avoid this, move your side-effect code into beforeMount or mounted instead.

Signed-off-by: [email protected]

[codecov-io]

Codecov Report

Merging #572 into dev will decrease coverage by 0.21%.
The diff coverage is 86.66%.

@@            Coverage Diff             @@
##              dev     #572      +/-   ##
==========================================
- Coverage   96.38%   96.17%   -0.22%     
==========================================
  Files          10       10              
  Lines         775      784       +9     
==========================================
+ Hits          747      754       +7     
- Misses         28       30       +2

Impacted Files	Coverage Δ
src/mixin.js	94.52% <86.66%> (-2.36%)	⬇️

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[kazupon]

Thank you for your PR!
Good catch!
SSR docs is correct.