export selector and handles fields from PDispatcher to support third-party extensions

[michaelsbradleyjr]

While working on the nim-task-runner repo, which imports nim-chronos, it was found that the selector and handles fields on PDispatcher needed to be exported so that objects returned from getThreadDispatcher() can have those fields accessed.

[cheatfate]

I understand that you need something to work, but explanation is also needs to be more comprehensive. Because currently i dont see any reasons to accept your PR, because it could lead to crashes and undefined behavior if i allow third-party extensions to modify this data structures.

[jakubgs]

I guess this change has no chance of getting in? Because currently nim-task-runner depends on the branch from this PR. Which means I can't update nim-task-runner to use the origin rather than the fork: https://github.com/status-im/nim-task-runner/pull/16

Which means I this tangled dependency mess for status-desktop described in: https://github.com/status-im/status-desktop/pull/7740#issuecomment-1267212386

[jakubgs]

Based on comment from @iurimatias the Desktop team has no intention to upgrade nimbus-build-system, so this might not become an issue in the long term, but the fact remains that status-desktop does use two versions of nim-chronos in their submodules.

[arnetheduck]

replaced with #363 which is the actual version used right now