No longer relevant after #5957. This hack existed because of another
hack where the numthreads parameter, on regtest, doubled as how many
blocks to generate.
To determine the default for `-par`, the number of script verification
threads, use [boost::thread::physical_concurrency()](http://www.boost.org/doc/libs/1_58_0/doc/html/thread/thread_management.html#thread.thread_management.thread.physical_concurrency)
which counts only physical cores, not virtual cores.
Virtual cores are roughly a set of cached registers to avoid context
switches while threading, they cannot actually perform work, so spawning
a verification thread for them could even reduce efficiency and will put
undue load on the system.
Should fix issue #6358, as well as some other reported system overload
issues, especially on Intel processors.
The function was only introduced in boost 1.56, so provide a utility
function `GetNumCores` to fall back for older Boost versions.
Change the few occurrences of the deprecated `auto_ptr` to c++11 `unique_ptr`.
Silences the deprecation warnings.
Also add a missing `std::` for consistency.
