Fix time_t serialization issue

On 32-bit MinGW-w64, time_t is int32_t.  The existing code was serializing
time_t directly and implicitly assuming that time_t is int64_t.  This commit
formalizes that assumption by serializing int64_t directly and casting to
time_t where appropriate.

Thanks go to greatwolf for reporting this issue.

monero-project/bitmonero#88