Change new constants in txdb.h to int64_t #3703

A shift overflow was happening when using these to check against in init.cpp. Fixes #3702.

Maybe it'd be better to change the constants to bytes (move the shifts to the constants instead of at the comparison site in init.cpp). Showing in MiB is only UI/commandline-level. Regardless, you need shifts at either place.

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I'm sorry this happened, but my compiler didn't give any warning. Also I'm not the one who introduced the shifts, that was sipa ;), I'm more comfortable with 4 * 1024 * 1024 for example.

ACK, makes clang happy.

@Diapolo Using multiplications instead of shifts would have resulted in the same problem. No hard feelings, this caught me unaware as well, in the 2010's we're casually veering outside the bounds of 32-bit types even for memory sizes.