Cache DecimalFormatSymbols instances to reduce lock contention

Constructing new DecimalFormatSymbols instances requires locking, and
in some sample Rails apps this contention was bubbling up as a
bottleneck. So, this change caches DecimalFormatSymbols instances
per-thread just like NumberFormat instances are already cached.

The overall throughput in my test application wasn't much changed
because of ActiveRecord's connection mutex still sitting in this way,
but this does get rid of one source of multi-threaded lock contention.