Document known issues with compressed InnoDB tables

There is a number of XtraBackup-related issues with compressed InnoDB tables. Those issues result from either server-side bugs, or OS configuration and thus, cannot be fixed on the XtraBackup side. However, we want to document them to at least make users aware of possible issues and workarounds. I'm not sure what part of the PXB manual this belongs, how about a new "Known issues and limitations" section?

Issues:

- For MySQL or Percona Server versions 5.1 and 5.5 there are known and unfixed bugs with redo-logging of updates to compressed InnoDB tables. For example, internal Oracle bug #16267120 has been fixed only in MySQL 5.6.12, but not in 5.1 or 5.5. The bug is about compressed page images not being logged on page reorganization and thus, creating a possibility for recovery process to fail in case a different zlib version is being used when replaying a MLOG_ZIP_PAGE_REORGANIZE redo log record. (NOTE: we can drop Percona Server from this paragraph if and when bug #1355704 is fixed);

- For MySQL or Percona Server version 5.6 it is NOT recommended to set innodb_log_compressed_pages=OFF for servers that use compressed InnoDB tables are backed up with XtraBackup. This option makes InnoDB recovery (and thus, backup prepare) sensible to zlib versions. In case the host where a backup prepare is performed uses a different zlib version than the one that was used by the server during runtime, backup prepare may fail due to differences in compression algorithms.