Use Cronos instead of NCrontab

[aidmsu]

This PR fixes #494, #567 by using Cronos instead of NCrontab in RecurringJobScheduler.

Pros of using Cronos:

• time zones support out of the box;
• correct Daylight Saving Time handling;
• support of non-standard characters: L, W, #, ?;
• performance: it's really very fast.

It means that:

• recurring jobs won't be missed on standard time (ST) to Daylight Saving Time (DST) transitions (when the clock jumps forward);
• minutely, hourly jobs won't be missed on DST to ST transitions (when the clock jumps backward);
• daily, monthly jobs won't be repeated on DST to ST transitions;
• users can schedule recurring jobs more flexible using non-standard characters. Example for last weekday of month: RecurringJob.AddOrUpdate(() => SomeMethod(), "* * LW * *")

Read more about all features.

[codecov-io]

Codecov Report

Merging #853 into dev will decrease coverage by 0.18%.
The diff coverage is 81.81%.

Impacted Files	Coverage Δ
src/Hangfire.Core/RecurringJobManager.cs	93.44% <100%> (ø)	⬆️
src/Hangfire.Core/Server/RecurringJobScheduler.cs	83.33% <80%> (-2.21%)	⬇️

[odinserj]

Finally we'll get around all the problems with Cron. Almost all, seconds still aren't supported yet 😉 Merging this awesome PR!

[patnolan]

Can you provide any guidance around when seconds will be supported?

I'm trying to make a decision between Quartz.net and Hangfire but the main requirement is the support for a trigger with a 45sec interval.

[tanushshukla]

Looks like 1.7.0 beta build is indeed using Cronos.

Thanks a lot! I'll be testing it out.