Stop using C++ auto to assign results of arithmetic operations

[ibc]

Details

• Fixes FeedbackRtpTransport.cpp may have an overflow mistake #1385
• We MUST NOT use auto to assign the result of var1 - var2. Despite what the type of var1 and var2 are, the compiler may choose a different type for the resulting vartiable. See below for a real example.
• I've also changed some other auto usages (those related to RTC::SeqManager<XXX>::IsSeqXxxxThan()). But probably there are more.
• My opinion is that we should STOP using auto.
• Also, using uint16_t result = var1 - var2 looks way better than auto result = static_cast<uint16_t>(var1 -var2), espite the latter makes sense sometimes when casting in operands is needed.

Real Example

#include <cinttypes>  // PRIu64, etc
#include <cstddef>    // size_t
#include <cstdint>    // uint8_t, etc
#include <cstdio>

int main() {

    uint16_t sequenceNumber = 0;
    uint16_t latestSequenceNumber = 65535;

    auto missingPackets = sequenceNumber - (latestSequenceNumber + 1);

    printf("missingPackets: %" PRIu16 "\n", missingPackets);

    return 0;
}

missingPackets: 4294901760

#include <cinttypes>  // PRIu64, etc
#include <cstddef>    // size_t
#include <cstdint>    // uint8_t, etc
#include <cstdio>

int main() {

    uint16_t sequenceNumber = 0;
    uint16_t latestSequenceNumber = 65535;

    uint16_t missingPackets = sequenceNumber - (latestSequenceNumber + 1);

    printf("missingPackets: %" PRIu16 "\n", missingPackets);

    return 0;
}

missingPackets: 0