Fixes for updating indices, charset/collation and engine type on plugin-created tables.

Under MySQL, new tables will be created as InnoDB with UTF-8 (utf8/utf8_bin) same as core tables.
Existing plugin tables will have table engine and default charset/collation updated, and string columns will have charset updated, at checkschema time.

Switched from 'DESCRIBE' to INFORMATION_SCHEMA for pulling column information in order to get charset. A second hit to INFORMATION_SCHEMA is also needed to get table properties.

Indices were only being created at table creation time, which ain't so hot. Now also adding/dropping indices when they change.

Fixed up some schema defs in OStatus plugin that were a bit flaky, causing extra alter tables to be run.

TODO: Generalize this infrastructure a bit more up to base schema & pg schema classes.

lib/schema.php

@@ -485,3 +485,9 @@ class Schema
         return $sql;
     }
 }
+
+class SchemaTableMissingException extends Exception
+{
+    // no-op
+}
+