openssl: abstract out OPENSSL_cleanse

This makes it easier for us to replace it if desired, since it's now only in
one spot. Also, it avoids the openssl include from allocators.h, which
essentially forced openssl to be included from every compilation unit.

src/base58.cpp

@@ -172,7 +172,7 @@ bool CBase58Data::SetString(const char* psz, unsigned int nVersionBytes)
     vchData.resize(vchTemp.size() - nVersionBytes);
     if (!vchData.empty())
         memcpy(&vchData[0], &vchTemp[nVersionBytes], vchData.size());
-    OPENSSL_cleanse(&vchTemp[0], vchData.size());
+    memory_cleanse(&vchTemp[0], vchData.size());
     return true;
 }