Merging #4145 broke the compilation on macOS

[rex4539]

mempool_limit.cpp:154:20: error: no matching function for call to 'max'
    int64_t cost = std::max(memUsage, MIN_TX_COST);
                   ^~~~~~~~
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../include/c++/v1/algorithm:2529:1: note: candidate template ignored: deduced conflicting types for parameter '_Tp' ('unsigned long' vs. 'unsigned long long')
max(const _Tp& __a, const _Tp& __b)
^
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../include/c++/v1/algorithm:2539:1: note: candidate template ignored: could not match 'initializer_list<type-parameter-0-0>' against 'unsigned long'
max(initializer_list<_Tp> __t, _Compare __comp)
^
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../include/c++/v1/algorithm:2521:1: note: candidate function template not viable: requires 3 arguments, but 2 were provided
max(const _Tp& __a, const _Tp& __b, _Compare __comp)
^
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../include/c++/v1/algorithm:2547:1: note: candidate function template not viable: requires single argument '__t', but 2 arguments were provided
max(initializer_list<_Tp> __t)
^
1 error generated.
make[2]: *** [libbitcoin_server_a-mempool_limit.o] Error 1

[Eirik0]

Thanks for reporting this. We will look in to it shortly.