Remove use of `__buitin_memset` in net/http ebpf

[damemi]

Ah I think it's the use of __buitin_memset which can only clear up to 512 bytes. I think to workaround this problem we can avoid using memset and manually set each scalar field to 0 and each character array 0th byte to '\0'. So essentially, manually initialize all fields. It should be less operations too, since we'll be setting a lot less bytes to 0, e.g. maybe 30?

Perhaps this shouldn't hold up the PR, we can follow-up with another change if we wanted to increase the sizes.

Originally posted by @grcevski in #790 (comment)

The url.full attribute for our net/http instrumentation will not fully parse long fields (such as host, path, or query) due to the issue in the linked comments.