Add limit operator for delete query for MySQL

[xartyomkax]

Adds the possibility to limit DELETE query for MySQL.

From MySQL doc

DELETE [LOW_PRIORITY] [QUICK] [IGNORE] FROM tbl_name [[AS] tbl_alias]
[PARTITION (partition_name [, partition_name] ...)]
[WHERE where_condition]
[ORDER BY ...]
[LIMIT row_count]

[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 40.00000% with 6 lines in your changes missing coverage. Please review.

Project coverage is 74.01%. Comparing base (353266c) to head (7152ebd).
Report is 7 commits behind head on master.

Files with missing lines	Patch %	Lines
src/Propel/Runtime/Adapter/Pdo/MysqlAdapter.php	0.00%	3 Missing ⚠️
...e/ActiveQuery/SqlBuilder/DeleteQuerySqlBuilder.php	66.66%	2 Missing ⚠️
src/Propel/Runtime/Adapter/Pdo/PdoAdapter.php	0.00%	1 Missing ⚠️

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Additional details and impacted files

@@             Coverage Diff              @@
##             master    #2054      +/-   ##
============================================
+ Coverage     68.56%   74.01%   +5.45%     
- Complexity     8073     8080       +7     
============================================
  Files           232      232              
  Lines         24569    24584      +15     
============================================
+ Hits          16845    18196    +1351     
+ Misses         7724     6388    -1336     

Flag	Coverage Δ
5-max	74.01% <40.00%> (+5.45%)	⬆️
7.4	74.01% <40.00%> (+5.45%)	⬆️
agnostic	?
mysql	69.84% <40.00%> (?)
pgsql	69.89% <40.00%> (?)
sqlite	67.82% <40.00%> (?)

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