Rename master branches for ReactPHP projects #453

I'm suggesting we rename the master branches like already done in reactphp/async, reactphp/filesystem and reactphp/promise.

This would be the case for the following projects:

• in reactphp/cache into 1.x
• in reactphp/child-process into 0.7.x
• in reactphp/datagram into 1.x
• in reactphp/dns into 1.x
• in reactphp/event-loop into 1.x
• in reactphp/http into 1.x
• in reactphp/pomise-stream into 1.x
• in reactphp/pomise-timer into 1.x
• in reactphp/socket into 1.x
• in reactphp/stream into 1.x

As @clue already described in reactphp/promise#212:

This has originally been brought up in reactphp/promise#204 by @Seldaek, but instead of defining a branch alias, this PR suggests simply renaming what is currently master to 3.x. This is in line with our existing 2.x and 1.x branches and means we do not have to update any alias definitions once we would start working on a potential future 4.x.

From a consumer's perspective, this means installation is now much easier, as targeting ^3@dev (or 3.x-dev) is semantically more obvious and safer than using dev-master. Additionally, keep in mind that branch names are development artifacts that are subject to change at some point in the future. Once we would decide to rename or delete any of our version branches, the ^3@dev reference would still be valid as it would also match stable versions if we have any ^3 tags at this point (which is safe to assume at this point).