Fix for valgrind-discovered trunk errors.

Fix for valgrind-discovered trunk errors.

During start-up, Valgrind reported many errors with a similar
message: "Use of uninitialised value of size 8...". These errors
were caused by HttpRequestMethod& parameter during private
key generation: it was used as a raw void* memory there. HttpRequestMethod
is a non-POD type and has some "padding" bytes, which are not initialized
by the constructor.

The patch simplifies private key generation by using a simple counter for this,
which is sufficient to create a unique key within a single Squid process.
Except fixing Valgrind errors, this solution saves many CPU cycles wasted on
generating MD5 hashes just to "guarantee" uniqueness within a single process.