enable table "connection" string with table partitioning feature.
support extra information "HA_EXTRA_WRITE_CAN_REPLACE" and
"HA_EXTRA_WRITE_CANNOT_REPLACE" for table partitioning feature.
support NULL for string variable with PLUGIN_VAR_MEMALLOC.
All storage engine can use table "connection" string with table
partitioning feature by using this patch. Currently table "connection"
string is ignored by overwriting partition "connection" string. It is
for federatedx storage engine. But it should be implemented using
another way, because current implementation causes confusion for
All storage engine can get extra information
"HA_EXTRA_WRITE_CAN_REPLACE" and "HA_EXTRA_WRITE_CANNOT_REPLACE" threw
table partitioning feature by using this patch.
Avoid to call my_free() using null pointer when string variable is
changed from null by using this patch.