[refactor] introduce debug classes under new structure

[georgehristov]

Split of #196

Part I - Refactoring of Debug proxy connections

[mvorisek]

I propose to write here: "will be removed in 2020-dec"

It is much easier for users to know when it will be removed and when to remove it in the code.

(update in everywhere)

[mvorisek]

Seems ok to merge except one thing - Driver vs. Connection.

Why is driver really that much better?

Driver is usually used for mysqli, PDO, so Connection seems here ok for me to keep and to keep all consistency.

Even if really needed, I propose to keep Connection for now and rename everything at once after the new structure is set. Why? It is much easier to refactor purely NS in user's projects and later (after another consistent release) refactor the short name.

@georgehristov ok with you?

[mvorisek]

Then feel free to rename NS of other engines. But please, do not copy from old PR, only minimalistic rename, I will check (still have to compare manually because of old classes are not removed completely so git does handle this as renames).

[georgehristov]

I would keep this PR only for the Debug connections.
You are right that the more concentrated is the PR the easier to understand and accept.
Refactoring of NS structure will follow in another PR.

[georgehristov]

@mvorisek done so pls approve for the record. I will not merge

[mvorisek]

Pure NS updates/renames are done well. Approving rename of Debug Connections. No unresolved feedback left.

Please rename freshly (do not reuse anything from changed code from your huge PR) other engines.

[mvorisek]

@georgehristov I checked the changelog after rel 2.1.0 and we can merge this right away. If ok with you, merge this PR and create another one for the pure renames (with BC classes like here). @romaninsh is slow, we can merge all renames today today.

[georgehristov]

Good. Moving on to next task now ...