Allow overriding implicit indexes

[DBAL-1106] Improve schema introspection performance on Oracle

[DBAL-1104] Add support for object hydration in oci8 driver

Update MysqliStatement to support PDO::FETCH_OBJ

Close #755

Do not TRIM() parentheses around partial indexes conditions

Close #716

Because PostgreSQL will return the expression with a lot of
parentheses we cannot TRIM() them easily. It is easier and more
correct to adapt to what PostgreSQL returns. That means the declaration
of partial indexes must be updated as follow:

Before:
``options={"where": "other_id IS NULL"}``

After:
``options={"where": "(other_id IS NULL)"}``

And fore more complex conditions, that would be:
``options={"where": "(((id IS NOT NULL) AND (other_id IS NULL)) AND (name IS NULL))"}``

Type json_array is not consistent with NULL values

[DBAL-1082] Fix SchemaTool does not generate SQL for MySQL unsigned float

Close #749

[DBAL-1063] Allow defining duplicate indexes on a table

PostgreSQL needs explicitly closed connection in functional test

Fixed typo

[DBAL-1097] Fix foreign key constraint referential action on Oracle

[DBAL-1087] Length of fixed string type (char) is ignored on Postgre schema update

Close #751

Fix error handling in mysqli driver

[DBAL-1095] Fix index introspection on SQL Anywhere

Fix creating and dropping database on PostgreSQL

Cleanup travis database creation