Update SQLConf.scala

use concurrent.ConcurrentHashMap instead of util.Collections.synchronizedMap
@rxin I think the performance distinction is extremely minor in this case, as there is only one ConcurrentHashMap. ConcurrentHashMap's API tends to be nicer to use, though, as people may not realize that iteration over a SynchronizedMap is not threadsafe, like in the current implementation of SQLConf.

As @baishuo mentioned, if we use synchronizedMap we'll have to add settings.synchronized {} in a few places now.