Switch all RNG code to the built-in PRNG.

It includes the following policy changes:
* All GetRand* functions seed the stack pointer and rdrand result
  (in addition to the performance counter)
* The periodic entropy added by the idle scheduler now seeds stack pointer,
  rdrand and perfmon data (once every 10 minutes) in addition to
  just a sleep timing.
* The entropy added when calling GetStrongRandBytes no longer includes
  the once-per-10-minutes perfmon data on windows (it is moved to the
  idle scheduler instead, where latency matters less).

Other changes:
* OpenSSL is no longer seeded directly anywhere. Instead, any generated
  randomness through our own RNG is fed back to OpenSSL (after an
  additional hashing step to prevent leaking our RNG state).
* Seeding that was previously done directly in RandAddSeedSleep is now
  moved to SeedSleep(), which is indirectly invoked through ProcRand
  from RandAddSeedSleep.
* Seeding that was previously done directly in GetStrongRandBytes()
  is now moved to SeedSlow(), which is indirectly invoked through
  ProcRand from GetStrongRandBytes().

src/random.h

@@ -13,11 +13,13 @@
 #include <stdint.h>
 #include <limits>
 
-/* Seed OpenSSL PRNG with additional entropy data */
-void RandAddSeed();
-
 /**
- * Functions to gather random data via the OpenSSL PRNG
+ * Generate random data via the internal PRNG.
+ *
+ * These functions are designed to be fast (sub microsecond), but do not necessarily
+ * meaningfully add entropy to the PRNG state.
+ *
+ * Thread-safe.
  */
 void GetRandBytes(unsigned char* buf, int num);
 uint64_t GetRand(uint64_t nMax);
@@ -25,21 +27,26 @@ int GetRandInt(int nMax);
 uint256 GetRandHash();
 
 /**
- * Add a little bit of randomness to the output of GetStrongRangBytes.
- * This sleeps for a millisecond, so should only be called when there is
- * no other work to be done.
- */
-void RandAddSeedSleep();
-
-/**
- * Function to gather random data from multiple sources, failing whenever any
- * of those sources fail to provide a result.
+ * Gather entropy from various sources, feed it into the internal PRNG, and
+ * generate random data using it.
+ *
+ * This function will cause failure whenever the OS RNG fails.
+ *
+ * Thread-safe.
  */
 void GetStrongRandBytes(unsigned char* buf, int num);
 
+/**
+ * Sleep for 1ms, gather entropy from various sources, and feed them to the PRNG state.
+ *
+ * Thread-safe.
+ */
+void RandAddSeedSleep();
+
 /**
  * Fast randomness source. This is seeded once with secure random data, but
- * is completely deterministic and insecure after that.
+ * is completely deterministic and does not gather more entropy after that.
+ *
  * This class is not thread-safe.
  */
 class FastRandomContext {