Migrate HttpRequest to use Env's library loading.

Details:
- Switch to static initialization as a better alternative to RTLD_NODELETE and
  resolving symbols again.
- Change LoadLibrary to not return an error if a library has already been
  loaded. This isn't an error that callers will ever have to handle, because
  loading a library multiple times is ok (handles are ref-counted in glibc).
  This makes it not error if you're loading symbols from the process's global
  symbol table (useful if a lib is statically linked).
Change: 133313722