Change BIO_number_read and BIO_number_written() to be 64 bit

The return type of BIO_number_read() and BIO_number_written() as well as
the corresponding num_read and num_write members in the BIO structure has
been changed from unsigned long to uint64_t. On platforms where an unsigned
long is 32 bits (e.g. Windows) these counters could overflow if >4Gb is
transferred.

With thanks to the Open Crypto Audit Project for reporting this issue.

Reviewed-by: Richard Levitte <levitte@openssl.org>