Remove get lock 2 - rebased and extended a bit.#7
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get_lock() changes behaviour in MySQL 5.7, which causes in turn causes workers to lock more rows than necessary. This patch changes the behavior of Data::Consumer::MySQL2 to claim rows by doing SELECT followed by UPDATE, instead of two SELECTs (second select was necessary to validate result of GET_LOCK() since it wasn't atomic, and was a major change from Data::Consumer::MySQL). SELECT followed by UPDATE allows two claim strategies to be implemented, both relying on implicit database locks: atomic CAS (used by default) and SELECT FOR UPDATE.
Avoids stepping into a perl bug where fatal warning in a destructor causes infinite recursion somewhere in perl guts (seems to be fixed in modern versions).
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A rebased version of #5