Move initial TLS write record layer code into new structure

Move initial TLS write record layer code into new structure

The new write record layer architecture splits record writing into
a "write_records" call and a "retry_write_records" call - where multiple
records can be sent to "write_records" in one go. We restructure the code
into that format in order that future commits can move these functions into
the new record layer more easily.

Reviewed-by: Hugo Landau <hlandau@openssl.org>
Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/19198)