Reuse MAC writing logic in s2n_record_read

[goatgoose]

Problem:

#4539 added a new s2n_record_write_mac function which calculates the record MAC and writes it into a stuffer. This function was added to write the MAC into the record. However, a MAC also needs to be calculated when receiving stream and CBC records in order to compare the calculated MAC against the received MAC. s2n_record_write_mac can be reused for this purpose as well.

Solution:

Reuse s2n_record_write_mac when validating the record MAC:

• s2n_record_read_cbc
• s2n_record_read_stream

[goatgoose]

Unfortunately this change impacts the sidetrail proofs. When I attempted this it increased sidetrail's runtime by a lot. It might be difficult to implement this and work around all of the proofs.