Contains Spiral patches:
- Spiral Patch 007: 007_mariadb-10.2.0.partition_fulltext.diff  MDEV-7705
- Spiral Patch 038: 038_mariadb-10.2.0.partition_fulltext2.diff MDEV-7734

- This commit has the following differences compared to the original patches:
  - Added necessary full text search cleanup at the storage engine layer
    that was omitted in the original patch.
  - Added test case.
- A lot of code cleanups to make the code notable smaller.
- Changed SQL code to use ha_ft_end() instead of ft_end()

Original author: Kentoku SHIBA
First reviewer:  Jacob Mathew
Second reviewer: Michael Widenius