Introspect indexes from the database prior to evolutions.
The naming of indexes vary between databases and versions of Django,

and can also be different if indexes were manually modified. Because of

this, we can't just guess index names.
This is the first part toward a solution. A new create_database_sig

method, and accompanying database backend methods, introspect the

database prior to evolutions to find the list of tables and their

indexes. From this, a coming change will be able to look up the correct

index, given a table and list of fields.
All the evolution-related functions require a database signature now.

These signatures are not stored, but are used throughout an evolution

as a representation of the original and then modified state of important

parts of the database.
A unit test was added to check loading signature state.

Unit tests pass for sqlite3, MySQL, and Postgres.