This causes masterConn warnings in other places, like:
```
Expectation (masterConns <= 0) by SpecialPageFactory::executePath not met:
[connect to 10.64.16.7 (nlwiki)]
#0 /srv/mediawiki/php-1.29.0-wmf.7/includes/libs/rdbms/TransactionProfiler.php(160): TransactionProfiler->reportExpectationViolated()
#1 /srv/mediawiki/php-1.29.0-wmf.7/includes/libs/rdbms/loadbalancer/LoadBalancer.php(590): TransactionProfiler->recordConnection()
#2 /srv/mediawiki/php-1.29.0-wmf.7/includes/GlobalFunctions.php(3081): LoadBalancer->getConnection()
#3 /srv/mediawiki/php-1.29.0-wmf.7/extensions/CheckUser/CheckUser.hooks.php(161): wfGetDB()
#4 /srv/mediawiki/php-1.29.0-wmf.7/includes/Hooks.php(195): CheckUserHooks::updateCUEmailData()
#5 /srv/mediawiki/php-1.29.0-wmf.7/extensions/ContactPage/ContactPage_body.php(409): Hooks::run()
#6 /srv/mediawiki/php-1.29.0-wmf.7/includes/htmlform/HTMLForm.php(662): SpecialContact->processInput()
#7 /srv/mediawiki/php-1.29.0-wmf.7/includes/htmlform/HTMLForm.php(554): HTMLForm->trySubmit()
#8 /srv/mediawiki/php-1.29.0-wmf.7/includes/htmlform/HTMLForm.php(569): HTMLForm->tryAuthorizedSubmit()
#9 /srv/mediawiki/php-1.29.0-wmf.7/extensions/ContactPage/ContactPage_body.php(229): HTMLForm->show()
#10 /srv/mediawiki/php-1.29.0-wmf.7/includes/specialpage/SpecialPage.php(522): SpecialContact->execute()
#11 /srv/mediawiki/php-1.29.0-wmf.7/includes/specialpage/SpecialPageFactory.php(577): SpecialPage->run()
#12 /srv/mediawiki/php-1.29.0-wmf.7/includes/MediaWiki.php(283): SpecialPageFactory::executePath()
#13 /srv/mediawiki/php-1.29.0-wmf.7/includes/MediaWiki.php(858): MediaWiki->performRequest()
#14 /srv/mediawiki/php-1.29.0-wmf.7/includes/MediaWiki.php(519): MediaWiki->main()
#15 /srv/mediawiki/php-1.29.0-wmf.7/index.php(43): MediaWiki->run()
#16 /srv/mediawiki/w/index.php(3): include()
#17 {main}
```
In this case, SpecialContact is a special page that's not marked as doing writes, and so SpeicalPageFactory imposes the expectation that no master connection will be opened. SpecialContact doesn't try to do any writes, but it runs the EmailUser hook, and CheckUser hooks into that and performs a DB write.
I'm guessing we should defer the write in updateCUEmailData() maybe? But I'm not sure.