feat: add getting trigger checks and merge functions

[almostinf]

Add getting trigger checks and merge functions

Add additional functions needed for PR #946:

• getTriggersLastCheck function, with which I will be able to get checkData for each trigger passed in the triggerIDs array, if no checkData was found for the passed triggerID, then checkData = nil
• generic function MergeToSorted, which takes as input 2 arrays that can be compared by Less function, then merges them into one sorted array

Divided 1 big PR into 3 parts, here are the rest:

• feat(notifier): fix sending unwanted notifications on muted and deleted triggers and muted metrics #943
• feat(notifier, api): fix sending notifications on muted and deleted triggers #946

[codecov-commenter]

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@@                        Coverage Diff                        @@
##             feat/add-redis-notifier-config     #944   +/-   ##
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