Add better handling of deadlocks in DbScheduler

[Xnopyt]

This updates DbScheduler to handle deadlocks better when claiming jobs. This should prevent potential issues when running multiple workers for the MonitorCommand.

This makes the following changes:

• Restructures the query to SELECT ... FOR UPDATE and the update the rows as claimed, which should prevent unnecessary locks when indexes are properly configured for the table.
• Optionally allows SKIP LOCKED to be used on the query. This change allows jobs to be considered claimed by a worker as soon as the row becomes locked.
• Support for automatic retry with backoff if a deadlock does occur. This uses stechstudio/backoff library and requires a Backoff object to passed into the constructor.

[codecov]

Codecov Report

❌ Patch coverage is 97.87234% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 68.07%. Comparing base (85f37a1) to head (51b5aaa).
⚠️ Report is 5 commits behind head on 2.x.x.

Files with missing lines	Patch %	Lines
src/Scheduler/DbScheduler.php	97.87%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##              2.x.x      #15      +/-   ##
============================================
+ Coverage     65.43%   68.07%   +2.63%     
- Complexity      120      132      +12     
============================================
  Files            10       10              
  Lines           434      473      +39     
============================================
+ Hits            284      322      +38     
- Misses          150      151       +1     

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[chrisminett]

Does it need to use an extra dependency for retries? Seems like you've got all the logic here already, apart from the basic while() and counter. Retries could then just be an optional parameter ($maxAttempts = 1) with no need for passing in a dependency or having a different method.

Also, looks like the examples in README.md and /examples may need updating for the changes to DbScheduler.

[Xnopyt]

@chrisminett Examples and README have been updated with the new usage of DbScheduler

[amaanhassim]

should we be binding batchSize just to be safe?

Limit :batchSize
and then add the batchSize through the stuff below where you are doing minimumPickup

[Xnopyt]

It's not quite that simple. The default type PDO uses for binding is string (which would be a syntax error here as it would be quoted) and phlib/db does not support setting the type of a bind.
This should be safe as-is, we are using strict types and $batchSize is type hinted as an int, so it shouldn't be possible to pass an unsafe value here. The :minimumPickup binding was added back in the PHP 5.4 days before strict typing was in the language.