Create a new object database with no backends

Create a new object database and automatically add the two default backends:

Add a custom backend to an existing Object DB

Close an open object database

Read an object from the database

Read the header of an object from the database, without reading its full contents

Write an object to the database

Determine if the given object can be found in the object database

Determine the object-ID (sha1 hash) of the given git_rawobj

Release all memory used by the obj structure