Deprecate gid parameter to db.xact(...)

[jwp]

The gid argument enabled a 2pc transaction. When given a gid, the transaction object required that the prepare() method be used before committing on exit. This was intended to provide an abstraction, but use in any distributed transaction manager is likely to lead to more pain than it's worth. For v1.0, the gid documentation will simply be removed, and in 1.1, we'll probably throw a deprecation warning if it's a provided. Subsequent versions will just throw the TypeError as the feature will not exist.

[jwp]

Maybe deprecate all parameters to xact()?

It may be better to just let them set the isolation level using with db.settings():...

[jwp]

Leave isolation and mode. They're convenient.

Deprecation of gid= has been noted in v0.9's documentation. Feature will no longer be documented in 1.0, and it will subsequently be removed in 1.1.

[jwp]

v1.0 branch has removed the documentation. Deprecation warnings are not being issued at this point. Subsequent bugfix releases may include warnings.