Make native integration easier

[tmasternak]

Goal

The goal of this task is to make changes to SqlServer table schema to make the native integration easier.
It groups other issues which have been created separately.
Those changes should be implemented in such a way that wire compatibility with older versions is preserved.
Potential changes:

• Make the Id column a IDENTITY instead of UniqueIdentifier. Details: Make the Id column a IDENTITY instead of UniqueIdentifier #74
• Make the headers column nullable. Details: Make the headers column nullable #73
• Make the Recoverable column default to true. Details Make the Recoverable column default to true #71

[MarcinHoppe]

Would the new schema apply only to endpoints created with transport with this feature implement or are we actually thinking about migrating schemas of existing queue tables?

[tmasternak]

@MarcinHoppe TBH I don't know :/. Ideally users should not care which means new endpoints just work with the old once. I know that one of the discussed options where updatable views but not decision was made.

I think that either way any changes to table schema should probably be postponed until we figure out the best approach to multi-instance support #139. One of the options there is to provide some kind on synchronization process and that might be a good moment to figure out possibilities for changing the schema.

[mauroservienti]

Closed in favor of #179 that will have to deal with how to evolve the schema.