Switch the handshake buffer from memory BIO to BUF_MEM.

A memory BIO is internally a BUF_MEM anyway. There's no need to bring
BIO_write into the mix. BUF_MEM is size_t clean.

Change-Id: I4ec6e4d22c72696bf47c95861771013483f75cab
Reviewed-on: https://boringssl-review.googlesource.com/5616
Reviewed-by: Adam Langley <agl@google.com>