Fix multitag cardinality bug

[yvanoers]

Fixes #821.

[vcastellm]

Good job here! thx @yvanoers, just one observation, I had a hard time reviewing because filterNodes function modifies execNodes inside, this is an unexpected change, I tend to prefer returning the modified structure for clarity, WDYT?

[yvanoers]

I made the function return everything it touches. Also reduced the side effect of the tags changing and now returning new value.
I didn't want to create a new map to eliminate the execNodes side effect, because it would hit performance and mem use.

To really properly fix the lengthiness of these functions, I would create a separate struct with accompanying functions and put it all in a separate file. I didn't want to go that way, though, because it seems like an extensive solution to a relatively small problem.

Anyway, I pushed some changes, have a look and let me know your thoughts, @Victorcoder !

[vcastellm]

This looks much more intuitive, thanks!

[vcastellm]

Hi @yvanoers I'm receiving reports of this change affecting the evenly balancing of executions in nodes

I did not have verified it myself, but I'm letting you know before looking at it, because if you see a quick fix.

[yvanoers]

@Victorcoder Interesting, I'll have a look tonight.
Am I correct in assuming that the graph represents the number of jobs executed on 3 different servers (a different color for each server)?

[vcastellm]

@yvanoers yes, correct, it changed colors because of node restart, switching leader, jobs are mostly executed on the leader

[yvanoers]

@Victorcoder I found the problem, I will create a pull request shortly.