random number generator for `a` doesn't comply with RFC 5054

RFC5054 says to use a minimum of 256 bit random numbers and to pick a number in the range [1,N). The current client lib uses exactly 256 bits. It would be better to use max(N.getLength(), 256) bit random BigInteger values and then mod(N). This would mean that for N of 1024 bits we would use 1024 random bits rather than the current 256 random bits.