Changed API of random module to use size_t

size_t is used as the type of the amount of generated bytes.

Also, fixed a typo on include guard.
Added test 01-internal 19 on CMakeLists.txt

The test 19 was commited but it was not included on CMakeLists.txt.
Fixed that.
Small improvement on onion_sessions_generate_id()

onion_random_generate is called only once to generate 32 random numbers.