Improve reflection mechanism and alembic interactions

[adrien-berchet]

Goals of this PR:

• keep the spatial index creation easy (with spatial_index=True) and compatible with all SQLAlchemy mechanisms and with Alembic.
• fix the reflection mechanism for spatial types and spatial indexes.
• fix Issue with PostGIS spatial index using 0.11 #376

Main issues:

• spatial indexes are properly reflected with Postgres but not with SQLIte (because they are tables).
• indexes are reflected after the columns so it is not possible to know during column processing if we should create an index or let it be reflected afterwards.
• using _column_flag=True makes it work with to_metadata but it's a private attribute so it might not be reliable?
• Alembic triggers the before_create and after_create events but not the before_drop and after_drop events on which we rely on.

[adrien-berchet]

Hi @zzzeek
Here is the following of sqlalchemy/alembic#1034.
As I said, I still have (at least) one issue with Alembic: the CreateTableOp operation triggers the before_create and after_create events as expected but the DropTableOp does not trigger the before_drop and after_drop events on which we rely on to properly drop the spatial columns and to clean the remaining indexes with SQLite. Is there any reason for that or was it forgotten? I can create an issue in Alembic for this.

[zzzeek]

the before/after create dispatch was added to support some ENUM use cases which relied upon these to create their constraints. it looks like the current scope of before/after create/drop is limited to just MetaData and Table in SQLAlchemy (like, not for constraints or anything like that) so sure if you want to send a PR w/ tests for the drop events, that's fine, look right here:

https://github.com/sqlalchemy/alembic/blob/188dd8b2f098e804c8e64956e9c6490d41f1d7ce/alembic/ddl/impl.py#L350-L374