dnsdist: Handle out-of-memory exceptions in the UDP receiver thread

[rgacogne]

Short description

Most exceptions are handled by handleOnePacket(), but we might be out of memory (std::bad_alloc) in which case we DO NOT want to log (as it would trigger another memory allocation attempt that might throw as well) but wait a bit (one millisecond) and then try to recover.

Checklist

I have:

• read the CONTRIBUTING.md document
• compiled this code
• tested this code
• included documentation (including possible behaviour changes)
• documented the code
• added or modified regression test(s)
• added or modified unit test(s)

[omoerbeek]

I was a bit puzzled by the while loop calling handleOnePacket not having a try-catch block and the (dynamic) nesting of try-catch blocks in the else branch.
Is it an idea to not have a try/catch block in handleOnePacket, but have it in the two call spots? IMO it would make the structure more clear as the two spots calling handleOnePacket would have the same structure.

[rgacogne]

Done!