Increase max number of threads threshold

This commit increases the lower bound of the max processes ulimit, to
prepare for a world where Elasticsearch instances might be running with
more the previous cap of 32 processors. With the current settings,
Elasticsearch wants to create roughly 576 + 25 * p / 2 threads, where p
is the number of processors. Add in roughly 7 * p / 8 threads for the GC
threads and a fudge factor, and 4096 should cover us pretty well up to
256 cores.

core/src/main/java/org/elasticsearch/bootstrap/BootstrapCheck.java

@@ -308,7 +308,8 @@ boolean isMemoryLocked() {
 
     static class MaxNumberOfThreadsCheck implements Check {
 
-        private final long maxNumberOfThreadsThreshold = 1 << 11;
+        // this should be plenty for machines up to 256 cores
+        private final long maxNumberOfThreadsThreshold = 1 << 12;
 
         @Override
         public boolean check() {