Test failed because it hit net_write_timeout. It might happen in
different circumstances, and that's not what the testcase tests,
so the timeout is now set to a bigger value.
engine-independent statistics.
When the primary key was dropped or changed statistics on secondary
indexes for the prefixes that included components of the primary
key was not removed from the table mysql.index_stats.
Also fixed: in the some cases when a column was changed statistics
on the indexes that included this column was not removed from the
table mysql.index_stats.
Also disabled the test mdev-504 for --ps-protocol.
