No time-out when attempting to drop a locked database

[gkunter]

Originally reported by: gkunter (Bitbucket: gkunter, GitHub: gkunter)

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Sometimes, MySQL database connections don't get closed correctly (probably if exceptions occur during corpus installations or queries, but that's not clear as yet). If the corpus using that database is requested to be removed by the corpus manager, the GUI freezes, because the MySQL connection that tries to drop the database is waiting for table metadata lock.

• Care should be taken that there are no open connections
• MySQL connections should time-out
• If the connection that tries to drop the database doesn't succeed after a specific amout of time, an exception should be raised.

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• Bitbucket: https://bitbucket.org/gkunter/coquery/issue/156

[gkunter]

Original comment by gkunter (Bitbucket: gkunter, GitHub: gkunter):

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Fix Issue #156

[gkunter]

Original comment by gkunter (Bitbucket: gkunter, GitHub: gkunter):

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Try to use as many disposable DB connections as possible (see #156)