alembic/sqlalchemy: auto increment only allowed on a single column

alembic/sqlalchemy: auto increment only allowed on a single column

The extensions table defined two columns (id and priority) as primary key
autoincrement columns. However only one is allowed when defining the primary
key.

This patch removes the autoincrement attribute from the priority column since
it does not need to be as such and really should not have been on there in the
first place.

This patch also removes 'context', 'exten', and 'priority' from the primary key
index and creates a new combined unique contraint index on them.

ASTERISK-26183 #close

Change-Id: Ib9c712c612a4d7ec1edb0dcb77f1bae0905a470b
(cherry picked from commit f6ec94cca66addac71d566d6fa48188b407f26ba)