Summary
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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
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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
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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
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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>