Cache the siteidentifier and site contextid in local cache

Performance test

Before applying the patch

1. Turn on performance debugging in config.php:

   $CFG->perfdebug = 15;

2. Load any page, reload it a couple times to get everything warm and look at DB reads in the footer (eg I see 48)

After applying the patch

1. Apply the patch
2. Reload the same page several times
   1. Confirm that the DB reads went down by 2 (e.g. I see 46 instead of 48)

Upgrade test

1. Open the version.php file in your editor
2. Find the $version variable and increment it by one at the end
3. Run an upgrade in the UI (visit /admin)
   1. Confirm that there were no errors
4. Increment the version number in $version again
5. Perform a CLI upgrade:

   php admin/cli/upgrade.php

   1. Confirm that there were no errors

Installation test

1. Ensure that the code is up-to-date and clean:

   git checkout .

   git reset --hard origin/[branch]

   git clean -df

2. Uninstall your current site and remove the config.php
3. Perform a fresh install via the UI
   1. Confirm that there were no errors
4. Drop the database again but retain the config.php
5. Perform a fresh install via the UI
   1. Confirm that there were no errors
6. Uninstall your current site and remove the config.php
7. Perform a fresh install via the CLI using php admin/cli/install.php
   1. Confirm that there were no errors
8. Drop the database again but retain the config.php
9. Perform a fresh install via the CLI using php admin/cli/install_database.php
   1. Confirm that there were no errors