SVN Watcher/importer occasionally times out

[dd32]

Occasionally the plugin directory SVN watcher script times out while waiting for a response from the SVN server, when this happens an exception is thrown which triggers WordPress.org error handlers.

As the failure is often a transient network timeout, triggering a failure notification isn't ideal every time - instead, the failure notification should only be made if SVN is unresponsive for one or more requests.

[dd32]

In 5233:

Plugin Directory: SVN Watcher: Wait for 2 consequtive SVN failures to occur before throwing exceptions - a single network timeout is usually transitive.
This also switches cron tasks to be checked for / registered on init, to ensure that a failed recurring task (one which throws an exception) is re-registered in Cavalcade.

Fixes #2678

[dd32]

In 5234:

Plugin Directory: SVN Watcher: Discard automated plugin commits by plugin-master, these should not trigger plugin imports.
plugin-master is responsible for creating new plugin directories, and in the new directory adding the initial version of plugins.

See #2678

[dd32]

In 5235:

Plugin Directory: SVN Watcher: Don't re-register cron jobs on the init hook as it's too race-condititiony under load with Cavalcade.
Instead add a cron job to check the cronjobs are registered correctly, with a single event set to re-add svn watcher jobs when we know it's going to be marked as failed.

See #2678