SourceLayout: Move the Runner which manages SSL SessionCache to libsecurity

SourceLayout: Move the Runner which manages SSL SessionCache to libsecurity

Unfortunately the OpenSSL session cache callbacks cannot also be moved
due to circular dependency issues. However, when those are resolved by
later libsecurity API additions the callbacks will be much easier to
shift. For now the three symbols shared between the two libraries are
exposed by libsslsquid in the Ssl:: namespace.

Cache initialization is now moved into the Runner. Binding its state
initialization more tightly to the memory allocation and initialization.
Which also removes the need for explicit main.cc dependency.

One issue was uncovered during this:

* While ssl/support.h was defining a destruct_session_cache() function
that appeared to release the cache memory, it was not actually being
used anywhere. Which unless a fortuitous sequence of events is happening
means that the memory for the cache entries may not be released properly.
On the other hand the cache should only be erased on shutdown so the
effects of this are minor.

The unused function has been removed and the issue is now expicitly
noted in the Runner shutdown handling method for future investigation.