Allow version change transactions w/o modifying version number

[inexorabletash]

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 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 Backward-compatible schema changes are very hard #282
• Need concrete proposals (PR)

[sicking]

Would the proposal in #282 (comment) fix this?