Attempt to implement an in-memory store using SQLite

[k00ni]

What?

This PR contains code for an in-memory store which uses PDO + SQLite (:memory:). Because it is based on existing classes it provides the same functionality as its MySQL based pendant.

I also extended PHP-CS-Fixer coverage and improved code readability overall.

Details

• PDOAdapter class got another check to make sure it is only used with mysql protocol.
• Adapted classes like ARC2_Store to distinguish between MySQL and SQLite approaches
• ARC2_StoreLoadQueryHandler::checkSQLBuffers: removed fourth parameter, because it is unused inside the function
• ARC2_StoreSelectQueryHandler::getTempTableDef: renamed it to getTempTableDefForMySQL and added a second version for SQLite (called getTempTableDefForSQLite)
• reworked Github Actions to test the library (focused on simplifications and clear code)
• fixed coding styles in further files to improve code readability
• removed further instances of the error control operator @, e.g. @include_once (suppressing errors is bad coding style because you might not see when something went wrong)
• improved README.md (shows test cases, added in-memory example)
• added test coverage for MySQL 5.5

Ref: #105

[k00ni]

We have an in-memory store now! :rocket:

Added a small section to README.md about the new store: https://github.com/semsol/arc2/tree/pr/sqlite-in-memory-store#in-memory-store-sqlite-based

@semsol (or should I use @bnowack from now on?): What do you think? I know there are many changes, but if you could take a look at SQLite related changes in store/. My approach was to keep what you've built and add some if-clauses, in case SQL differs between MySQL and SQLite. Thanks in advance.

[k00ni]

@zozlak: What do you think about a standalone library providing an in-memory triple store? It could be based on this code here, using SQLite with :memory:.

[zozlak]

What do you think about a standalone library providing an in-memory triple store? It could be based on this code here, using SQLite with :memory:.

I think there are many things to discuss here but I would be glad to talk with you about it. I propose you contact me by email not to spam here too much.

[bnowack]

Wow! how cool is that?