RFC: Upgrade to RxJS 5.5

[tsvetomir]

RxJS 5.5 introduces a new way to import operators, namely pipeable operators née lettable operators. This new way to import operators improves module isolation and helps reduce bundle size.

Pipeable operators are only partially compatible with previous versions, particularly static methods like Obserbable.merge can lead to build errors when imported globally. See ReactiveX/rxjs#3222, #1214, #1230, #1236.

To remedy this situation, I propose to:

1. Update the peer dependencies of affected @progress/kendo-* packages to require "rxjs": "<5.5.0"
2. Release a major version of affected @progress/kendo-* packages that require "rxjs": "^5.5.0" and utilize pipeable operators.
3. Tag all Angular 4 LTS compatible package versions. You will be able to get the latest LTS-compatible version by specifying "ng4" as a version. Edit: No tags needed, packages remain compatible with Angular LTS, you just need to use rxjs > 5.5

Please note:

• Affected packages can be seen in the issue labels
• The previous major version will not receive updates and users are urged to update to rxjs 5.5 before updating to new Kendo UI package versions.

/cc @rusev @rkonstantinov @gyoshev @ggkrustev @tapopov @valchev @danielkaradachki @Raisolution

white space reserved for a rant on breaking changes in minor releases

[ggkrustev]

I do believe that upgrading to rxjs@^5.5.x is inevitable. I suppose that we can introduce a breaking change along with the Angular@6 release.

I am not sure how we should proceed with the Angular LTS release. Probably we should also have 2 versions of the packages. What do you think?