The memory of local temporary table can only be released when current connection is shutdown

## Bug Report
In TiDB, if we create a local temporary table, and insert some data into the table, then the memory took up by the temporary table will always be kept, even if we use `truncate` or `delete from temporary_table` to delete all the data in that table. The memory can only be released when current connection is shutdown.

The root cause is when `truncate` or `delete from temporary_table`, TiDB only call `kv.MemBuffer.Delete(key)` to delete the key, but in the implementation of `kv.MemBuffer.Delete(key)`, it actually set a tombstone value to the key instead of delete it. So the memory is not free after `truncate` or `delete from temporary_table`

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