BUILD: quic+h3: 32-bit compilation errors fixes

BUILD: quic+h3: 32-bit compilation errors fixes

In GH #1760 (which is marked as being a feature), there were compilation
errors on MacOS which could be reproduced in Linux when building 32-bit code
(-m32 gcc option). Most of them were due to variables types mixing in QUIC_MIN macro
or using size_t type in place of uint64_t type.

Must be backported to 2.6.