CRM-21568 - Move emptiness judgments from SettingsBag::setDb to InnoDBIndexer

CRM-21568 - Move emptiness judgments from SettingsBag::setDb to InnoDBIndexer

The `SettingsBag::setDb()` function calls any `on_change` listeners.  It
originally used "dumb on change" behavior (where it calls the listeners
without comparing values).  CRM-19610 had an issue where the `InnoDBIndexer`
was running a bit too often, so they tried to resolve it by
making the `SettingsBag::setDb()` more clever.

Unfortunately, that's been a bit bouncy, and the cleverness depends on one's
particular interpretation of 0 vs '0' vs '' vs NULL vs FALSE.

Before
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All on-change listeners may be skipped if there's particular mix of emptiness
in the old/new values.

After
-----
The on-change listeners always fire. However, the specific listener
involved with CRM-19610 will now ignore some insignificant changes.

Related discussion: https://github.com/civicrm/civicrm-core/pull/11417

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* CRM-21568:
  https://issues.civicrm.org/jira/browse/CRM-21568
* CRM-19610:
  https://issues.civicrm.org/jira/browse/CRM-19610