avoids duplicate deref calculations in useStoreDependency

[gmcnaughton]

cc @aem - another possible approach to #95

Avoids duplicate deref calculations in useStoreDependency. Only calculate the value once; use it to initialize state (if needed) and as the new value.

When the value changes, two deref calculations will still run:

1. initial render/hook call
2. re-render/hook call triggered by setting the new value in state

TODOs

• linter, checker, and test are passing
• any new public modules are exported from src/GeneralStore.js
• version numbers are up to date in package.json
• CHANGELOG.md is up to date