Avoid double-checked locking idiom; already using a ConcurrentHashMap.
A user reports a general Jenkins slowdown/hang in which one thread is locking the single builder
and for an unknown reason fails to proceed:
"Handling GET /job/…/api/python : …"
   java.lang.Thread.State: RUNNABLE
	at java.util.Arrays.copyOfRange(Arrays.java:2694)
	at java.lang.String.<init>(String.java:203)
	at java.lang.StringBuilder.toString(StringBuilder.java:405)
	at java.lang.Class.toString(Class.java:150)
	at java.lang.String.valueOf(String.java:2854)
	at java.lang.StringBuilder.append(StringBuilder.java:128)
	at org.kohsuke.stapler.export.NotExportableException.<init>(NotExportableException.java:36)
	at org.kohsuke.stapler.export.Model.<init>(Model.java:69)
	at org.kohsuke.stapler.export.ModelBuilder.get(ModelBuilder.java:48)
	- locked <0x000000054260a928> (a org.kohsuke.stapler.export.ModelBuilder)
	at org.kohsuke.stapler.export.Property.writeValue(Property.java:224)
while all other threads handling REST calls block.
Merge pull request #16 from jglick/ModelBuilder-concurrency

Make ModelBuilder safely concurrent