'register' storage class specifier is deprecated and incompatible with C++17

[mweisgut]

The keyword register is used, which is removed since C++17. https://en.cppreference.com/w/cpp/keyword/register

In file included from ../third_party/BwTree/src/bwtree.cpp:14:
In file included from ../third_party/BwTree/src/bwtree.h:63:
../third_party/BwTree/src/bloom_filter.h:138:5: warning: 'register' storage class specifier is deprecated and incompatible with C++17 [-Wdeprecated-register]
    register size_t hash_value = value_hash_obj(value);
    ^~~~~~~~~
../third_party/BwTree/src/bloom_filter.h:173:5: warning: 'register' storage class specifier is deprecated and incompatible with C++17 [-Wdeprecated-register]
    register size_t hash_value = value_hash_obj(value);
    ^~~~~~~~~
2 warnings generated.