INT-3316 BeanFactoryTypeConverter Concurrency

JIRA: https://jira.spring.io/browse/INT-3316

The `SimpleTypeConverter` delegate uses property editors
which are not thread safe. We protect against concurrent
access when the source is not a String, but when a String,
we delegate to the `STC`.

Synchronize access to the `STC.convertIfNecessary()` method.