connectionmanager: improve portability of string to integer conversion

On Windows using MSC, unsigned long is 32 bits even on 64-bit systems, while unsigned long long is 64 bits.

This commit replaces the call to std::stoul with std::stoull which should be portable.

This will address the following Jami-client issue:
jami-client-qt#1419

Change-Id: I9f7998573f493f479b9f97bf5d56ef65b8b58f57