[3.1.2 backport] Prevent minor version downgrade

By default, Sync Gateway should prevent startup when a collection being targeted by the database contains metadata from a higher Sync Gateway version.

We should perform this check at database creation, update, and startup.

Implementation overview:

• add Sync Gateway version to base.SyncInfo
• modify SetSyncInfo to populate version
• modify InitSyncInfo to populate version on init, and to identify version mismatch for existing files

Database startup

• When the SyncInfo in a collection targeted by a db is on a newer minor version, fail to load the database. (note: unlike existing handling for metadataID mismatches, we don't just want to leave it offline - should fail to load and report an error)
• It should be possible for users to delete or update the database config (to remove references to collections with a higher SyncInfo)

Database creation/update

• Write a new GetSyncInfo that does a simple fetch of Sync Info
• On creation/update, iterate over all collections targeted by the database, fetch SyncInfo, and return error if any of them have a newer major or minor version than SG's version

Config override

• We may want the ability for on prem customers to override and allow this downgrade, for cases where they know what they are doing. For this scenario I think it's reasonable to include an db-level unsupported flag that would disable the checks above

Testing recommendations

• All of the below should be tested with _default._default and named collections
• All of the above should be tested with
• bucket contains version with newer major version - should fail
• bucket contains version with newer minor version - should fail
• bucket contains version with newer maintenance version - should succeed
• bucket contains version with matching version - should succeed
• bucket contains version with older version - should succeed
• Test the following:
• Create db targeting collection(s)
• Update db to add collection
• Start db targeting collection(s)