Transfer the functionality from ssl3_read_n to the new record layer

Transfer the functionality from ssl3_read_n to the new record layer

This transfers the low level function ssl3_read_n to the new record layer.
We temporarily make the read_n function a top level record layer function.
Eventually, in later commits in this refactor, we will remove it as a top
level function and it will just be called from read_record.

Reviewed-by: Hugo Landau <hlandau@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/18132)