Name of tables and indexes conflicts for SQLite

Problem/Motivation

As discussed in #1008128: Do not use a single underscore as table and index separator on PostgreSQL and SQLite, naming collisions can occur between indexes and table names in SQLite and PostgresSQL.
That issue was closed in favor of #998898: Make sure that the identifiers are not more the 63 characters on PostgreSQL, but only the PostgresSQL part was fixed. This issue still affects SQLite

We wanted to add an index called search_api_language on the search_api_db_default_index resulting in the index name
search_api_db_default_index_search_api_language. This conflicted because we already have a table with that name.

I discovered this issue when fixing tests for Search API with SQLite enabled. SQLite gave us a conflicts because table names and index names have to be unique. They weren't and Drupal Core didn't solve this properly, as it does with the other SQL backends.

Proposed resolution

A patch attached in #2 implements the same fix as originally proposed in #1008128: Do not use a single underscore as table and index separator on PostgreSQL and SQLite.

Remaining tasks

Discuss, possibly write an upgrade path?

User interface changes

None

API changes

None

Data model changes

A double underscore is added to index names.