CCoinsKeyHasher::operator() should return size_t
CCoinsKeyHasher::operator() should return size_t

It currently returns uint64_t, which on older boost (at least 1.46) causes
test failures on 32-bit systems.

This problem was introduced in bc42503.

Fixes #4634.

(cherry picked from commit 6c23b082033b627f31170166c07ab35fa6be9343)

reddcoin-project/reddcoin,reddcoin-project/reddcoin,reddcoin-project/reddcoin,reddcoin-project/reddcoin,reddcoin-project/reddcoin,reddcoin-project/reddcoin