Per-domain directories: changes apply on the next request, and one broken directory doesn't block reloads (DIR-17, DIR-21)

A write to x:Directory or Authentication reloads the directories at once,
here and across the cluster. A directory that fails to open is logged as
a warning against its id and becomes unavailable; before, it was a build
error, and any build error stopped every later reload from applying.

per_domain_directory_tests covers acceptance tests 1, 3, 4, 6, 7, 9, 11,
15 and 19 over SQL directories on SQLite files: each domain against its
own directory and the default, no fallback to internal passwords, a
directory answering for another directory's domain, aliases and groups
dropped, recipients through the directory and a 4xx while it's down, app
passwords while it's down, password changes refused and then allowed
after a move to the internal directory, linked directories, tenant
foreign keys, and changes taking effect without a reload.

tests/src/directory/mod.rs

@@ -8,6 +8,8 @@ pub mod discovery;
 pub mod integration;
 pub mod ldap;
 #[cfg(feature = "sqlite")]
+pub mod per_domain; // inbuxa: per-domain directories
+#[cfg(feature = "sqlite")]
 pub mod sql;
 pub mod synchronization;
 pub mod unavailable;