Optimized BIO mem read - without reallocation

Currently on every BIO mem read operation the remaining data is reallocated.
This commit solves the issue.
BIO mem structure includes additional pointer to the read position.
On every read the pointer moves instead of reallocating the memory for the remaining data.
Reallocation accures before write and some ioctl operations, if the read pointer doesn't point on the beginning of the buffer.
Also the flag is added to rewind the read pointer without losing the data.

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