implement MySQL ON DUPLICATE KEY UPDATE

[Mike Bayer]

MySQL-specific variant implemented similarly to Postgresql's ON CONFLICT

original issue #960

[Mike Bayer]

https://gerrit.sqlalchemy.org/430

[Mike Bayer]

Implement MySQL's ON DUPLICATE KEY UPDATE

Added support for MySQL's ON DUPLICATE KEY UPDATE MySQL-specific :class:.mysql.dml.Insert object. Pull request courtesy Michael Doronin.

Co-authored-by: Mike Bayer [e-mail] Resolves: #4009 Change-Id: Ic71424f3c88af6082b48a910a2efb7fbfc0a7eb4 Pull-request: https://github.com/zzzeek/sqlalchemy/pull/365

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