Don't create temporary strings to escape DNSName labels

[rgacogne]

Short description

Shaves a nice 20%+ off DNSName::toString() for regular names, and cut in half the CPU time when we encounter a 'special' character (not . or \ and lesser or equal to 0x20 or greater or equal to 0x7f).

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)

[chbruyand]

LGTM, nice shave!

[chbruyand]

Maybe this would be worth to keep reserve()ing enough space to append the escaped label ?

[omoerbeek]

I think that is covered by line 172. In the regular case that should be enough.

[rgacogne]

In theory we should have reserved enough space already at the beginning of DNSName::toString(), but that might not be true due to escaping, I'll try to measure whether a ret.reserve(ret.size() + len) helps.

[chbruyand]

I think that is covered by line 172. In the regular case that should be enough.

Yes indeed, my bad.

[omoerbeek]

I think that is covered by line 172. In the regular case that should be enough.