Partially implements #74.
Securely erases keys from memory after they are no longer needed. Might have a
performance impact, which I haven't measured (perf measurements aren't
generally reliable on laptops).
Thanks to @stoffu for the suggestion to specialize the pod_to_hex/hex_to_pod
functions. Using overloads + SFINAE instead generalizes it so other types can
be marked as scrubbed without adding more boilerplate.
Keep the immediate direct deps at the library that depends on them,
declare deps as PUBLIC so that targets that link against that library
get the library's deps as transitive deps.
Break dep cycle between blockchain_db <-> crytonote_core.
No code refactoring, just hide cycle from cmake so that
it doesn't complain (cycles are allowed only between
static libs, not shared libs).
This is in preparation for supproting BUILD_SHARED_LIBS cmake
built-in option for building internal libs as shared.
