Pick up symbols from inside the .so where possible

[achamayou]

The recent PR to add snapshot download (#6727) highlighted the risk of external dependencies being picked up through cchost and being used inside the .so inadvertently. This change reverses the priority when resolving symbols, and links in snmalloc explicitly on the .so side (since it is no longer picked up).

       RTLD_DEEPBIND (since glibc 2.3.4)
              Place the lookup scope of the symbols in this shared
              object ahead of the global scope.  This means that a self-
              contained object will use its own symbols in preference to
              global symbols with the same name contained in objects
              that have already been loaded.

[achamayou]

==7144==You are trying to dlopen a ./liblogging.virtual.so shared library with RTLD_DEEPBIND flag which is incompatible with sanitizer runtime (see https://github.com/google/sanitizers/issues/611 for details). If you want to run ./liblogging.virtual.so library under sanitizers please remove RTLD_DEEPBIND from dlopen flags.

[achamayou]

This change would unfortunately prevent us from using ASAN, which is clearly not an acceptable tradeoff.
Another change we can explore is drop our vendored copy of nghttp2 in favour of the OS version, which may work, but is not guaranteed because Ubuntu 20.04 only ships with 1.40.
This would be resolved satisfactorily once we move to Azure Linux where the current release of nghttp2 is 1.61.