Templated Numerical Type

[chenhao-ye]

Support customized numerical type in the class template. In general, the numerical assumption is:

• To count the number of accesses, use size_t.
• To sum up the total memory of a key-value cache, use size_t. This should not be uint32_t because it can easily grow over 4 GB.
• The size of a single key-value pair, use uint32_t by default, but can be customized in the template as SizeType. A single key-value pair should not be larger than 4 GB, and we prefer to make the per-key metadata compacted.
• The type of hash is uint32_t by default, because we don't need as many bits, but it can be customized in the template as HashType.