Fix HTTP/3 frame prefix allocation

[xintenseapple]

Summary:

• Allocate HTTP/3 DATA and trailing HEADERS frame prefixes using exact QUIC varint length accounting, including reused-buffer capacity checks.

Verification:

• git diff --check; ./auto/configure --with-http_v3_module --with-http_ssl_module --without-http_rewrite_module --without-http_gzip_module --without-quic_bpf_module; make -j2

Fixes #21

[mdounin]

I don't think that trying to calculate exact size needed for the particular body buffer being sent is the way to go, especially because buffers are reused, so we have to additionally check the size of obtained free buffers before trying to use them. Also, too small NGX_HTTP_V3_VARLEN_INT_LEN value might result in issues elsewhere: for example, right now the usage of NGX_HTTP_V3_VARLEN_INT_LEN in ngx_http_v3_send_goaway() is safe, but that's mostly because there are three varint values in the buffer, and two of them are guaranteed to be small. A better approach would be to simply bump NGX_HTTP_V3_VARLEN_INT_LEN to 8.