Log Message:
Update databases/percona-toolkit to 2.2.18.

- pt-stalk now sorts the output of transactions by id
- Added "Shared" memory info to pt-summary
- Added the --no-vertical-format option for pt-query-digest,
  allowing compatibility with non-standard MySQL clients that
  don't support the \G directive at the end of a statement
- Fixed error when parsing tcpdump capture with pt-query-digest
- Improved pt-online-schema-change plugin documentation
- Clarified the description of the --attribute-value-limit option
  for pt-query-digest
- Fixed all PERL-based tools to return a zero exit status when run
  with the --version option
- Fixed error that sometimes prevented to choose the primary key
  as index, when using the -where option for pt-table-checksum
- Fixed the inability of pt-query-digest to parse the general log
  generated by MySQL (and Percona Server) 5.7 instance
- Clarified the description of the --verbose option for
  pt-slave-restart