[w3c/IndexedDB] Allow version change transactions w/o modifying version number (#304)

As discussed at TPAC 2019

* Use case: libraries (e.g. PouchDB) wanting to manage schema at a library level, not be forced to arbitrarily bump version.
* proposal: `open(DOMString name, optional [EnforceRange] unsigned long long version, optional DOMString smersion);` upgrade runs if _version_ is greater **or** _smersion_ doesn't match.
* _smersion_ is expected to be something like hash of database schema description; constrained to a [limited length string](https://github.com/heycam/webidl/issues/552) that's okay to cache in memory
* No, we're not serious about _smersion_ as a name 😆  It's short for "string version" or "semantic version".

Follow-ups needed:
* [ ] Reach out to libraries (e.g. PouchDB) and see if this is sufficient.
  * Concern: still doesn't allow creating new stores/indexes w/o closing all existing connections, e.g. see #282
* [ ] Need concrete proposals (PR)


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