There is no need to call `applyColumnTypes` more than once for the same
`databaseName` and `tableName`, it is just move the additional
`columnList` to the first call.
The given `columnLists` may not contain all columns available in the
given table. This patch prevents the code to fail. Before this patch the
code was panicking whenever it was trying to set a value into a `nil`
object, e.g. `columnList.GetColumn('non-existant').Type = SomeType`.
For some reason the application was not completely failing but as a
side-effect any column after the non-existant column would never get its
Type properly set.
Both Percona and Maria allow MySQL to be configured to listen on an extra port when their thread pool is enable.
* https://www.percona.com/doc/percona-server/5.7/performance/threadpool.html
* https://mariadb.com/kb/en/the-mariadb-library/thread-pool-in-mariadb-51-53/
This is valuable because if the table has a lot of traffic (read or write load), gh-ost can end up starving the thread pool as incomming connections are immediately blocked.
By using gh-ost on the extra port, MySQL locking will still behave the same, but MySQL will keep a dedicated thread for each gh-ost connection.
When doing this, it's important to inspect the extra-max-connections variable. Both Percona and Maria default to 1, so gh-ost may easily exceed with its threads.
An example local run using this
```
$ mysql -S /tmp/mysql_sandbox20393.sock -e "select @@global.port, @@global.extra_port"
+---------------+---------------------+
| @@global.port | @@global.extra_port |
+---------------+---------------------+
| 20393 | 30393 |
+---------------+---------------------+
./bin/gh-ost \
--initially-drop-ghost-table \
--initially-drop-old-table \
--assume-rbr \
--port="20395" \
--assume-master-host="127.0.0.1:30393" \
--max-load=Threads_running=25 \
--critical-load=Threads_running=1000 \
--chunk-size=1000 \
--max-lag-millis=1500 \
--user="gh-ost" \
--password="gh-ost" \
--database="test" \
--table="mytable" \
--verbose \
--alter="ADD mynewcol decimal(11,2) DEFAULT 0.0 NOT NULL" \
--exact-rowcount \
--concurrent-rowcount \
--default-retries=120 \
--panic-flag-file=/tmp/ghost.panic.flag \
--postpone-cut-over-flag-file=/tmp/ghost.postpone.flag \
--execute
```
