Mempool improvements, branch ID awareness
Whenever the local chain tip is updated, transactions in the mempool which commit to an
unmineable branch ID (for example, just before a network upgrade activates, where the
next block will have a different branch ID) will be removed.
Includes commits cherry-picked from the following upstream PRs:
- bitcoin/bitcoin#6654
- Only the mempool index change.
- bitcoin/bitcoin#6776
- bitcoin/bitcoin#7020
- bitcoin/bitcoin#6915
Part of #2074.
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.
