Extend JMap to support parallel heap dump

Summary

Add a new sub-command option "parallel" to the existing "dump" jmap command line option, which specifies how many threads to use to generate the heap dump file.

Problem

JDK-8239290 and related issues has introduced the parallel heap inspection, using the same mechanism for jmap -dump could help speed up heap dumping.

Solution

Add a new sub-command option "parallel" to the existing "dump" jmap command line option, which specifies how many threads to use to generate the heap dump file.

Specification

Updated specification.

Usage:
jmap -clstats <pid>
    to connect to running process and print class loader statistics
jmap -finalizerinfo <pid>
    to connect to running process and print information on objects awaiting finalization
jmap -histo[:[<histo-options>]] <pid>
    to connect to running process and print histogram of java object heap
jmap -dump:<dump-options> <pid>
    to connect to running process and dump java heap
jmap -? -h --help
    to print this help message

dump-options:
  live         dump only live objects
  all          dump all objects in the heap (default if one of "live" or "all" is not specified
  format=b     binary format
  file=<file>  dump heap to <file>
  gz=<number>  If specified, the heap dump is written in gzipped format using the given compression level.
               1 (recommended) is the fastest, 9 the strongest compression.
 + parallel=<number>  Number of parallel threads to use for heap dump:
 +                    0 (the default) means let the VM determine the number of threads to use.
 +                    1 means use one thread (disable parallelism).
 +                    For any other value the VM will try to use the specified number of threads, but might use fewer.

Example: jmap -dump:live,format=b,file=heap.bin,parallel=5 <pid>

histo-options:
  live             count only live objects
  all              count all objects in the heap (default if one of "live" or "all" is not specified)
  file=<file>      dump data to <file>
-  parallel=<count> generate histogram using this many parallel threads, default 0
-                     0 use system determined number of threads
-                     1 use one thread, i.e., disable parallelism
-                     n use n threads, n must be positive
+ parallel=<number>  Number of parallel threads to use for heap inspection:
+                     0 (the default) means let the VM determine the number of threads to use.
+                     1 means use one thread (disable parallelism).
+                     For any other value the VM will try to use the specified number of threads, but might use fewer.

Example: jmap -histo:live,file=/tmp/histo.data,parallel=5 <pid>