#196630 by yched: postpone warning message display until batch finished, so messages are not forgotten on non-JS batches

#199336 by JirkaRybka: remove improper t() use in update code (update goes all in English, but there were some mistakes), and add messages to updates where there was no return value otherwise, so users know what happens in each update

#194310 by chx, catch, KarenS: run updates for disabled but previously installed modules, if they are compatible with the current system

#200271 by catch: more old 4.6 to 4.7 update path functions removed, as we don't support direct upgrades from 4.6 to 6.x definitely

 - whitespaces at end of lines
 - indentation
 - control structure usage
 - whitespace in empty lines
 - phpdoc comment formatting

#141727 by merlinofchaos, dvessel, sun: restore themeability support for maintenance pages (regression)

- Patch #179143 by Gabor, JirkaRybka, chx, ChrisKennedy, et al: do not fire bootstrap hooks during update.

#181758 by webernet: move cache_block creation to update_fix_d6_requirements() to avoid warnings about the nonexistent table through the update process

#176805 by dww: conditional linking to dblog pages in update.php, so if the module is not enabled, we don't get broken links

#162432 by bjaspan and chx: serial columns need their keys defined in db_change_field() and friends in MySQL

#171646 by JirkaRybka: store version usage information for every source string, optimize caching and prune the initially unused strings

#170638 by JirkaRybka and chx: move update access variable to settings.php, so we can check whether it is wide open, and we have one place for settings

#154949 by Crell and dww: let modules and themes specify a minimum required PHP version, under which they will not be possible to be turned on

- Patch #146910 by dww: only allow enabling modules with the same Drupal core compatibility version.

- Patch #137211 by merlinofchaos: move theme information to .info files and improved theme inheritance.

#127539: progressive operation support, refactoring update.php code to a generic batch API to support runnning operations in multiple HTTP requests
  - update.php is already on the batch API
  - node access rebuilding is in the works
  - automatic locale importing is in the works

 Thanks to Yves Chedemois (yched) for the good code quality, very wide awareness of issues related to batches,
 and the fantastic turnaround times. Hats off.

- Refactor partial-page handling for the maintenance page. It's an exotic quirk for update.php and belongs there.

#100516: CSS preprocessor to cache and compress all .css files. Benchmarks show up to 40% faster page loads.