Make OpenSSL locking warnings more severe

GIT_BEGIN/END_DECL were missing from sys/refs.h and preventing
compilation with g++ as the symbol were mangled.

Our git_openssl_set_locking() would ideally not exist. Make it clearer
that we provide it as a last resort and you should prefer anything else.

Introduce stackable IO streams

We currently have gitno for talking over TCP, but this needs to know
about both plaintext and OpenSSL connections and the code has gotten
somewhat messy with ifdefs determining which version of the function
should be called.

In order to clean this up and abstract away the details of sending over
the different types of streams, we can instead use an interface and
stack stream implementations.

We may not be able to use the stackability with all streams, but we
are definitely be able to use the abstraction which is currently spread
between different bits of gitno.

This makes them show up in the reference, even if the text itself isn't
the most descriptive.

These have been found with

    grep -Przon '\n\ntypedef struct.*?\{' -- include
    grep -Przon '\n\ntypedef enum.*?\{' -- include

We have been refreshing on read and write for a while now, so
git_config_refresh() is at best a no-op, and might just end up wasting
cycles.

Introduce reference transactions