mysql: do not use unsafe TRUNCATE statement

TRUNCATE was chosen for performance reasons, but it's usually done on small
tables, and not for performance-critical operations. TRUNCATE commits
implicitely, so for pt/ttrans in particular, it's certainly slower due to extra
fsyncs to disk.

On the other side, committing too early can corrupt the database if the storage
node is stopped just after. For example, a failure in changePartitionTable()
can cause 'pt' to remain empty.
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