Convert dtls_write_records to use standard record layer functions

Convert dtls_write_records to use standard record layer functions

We have standard functions for most of the work that dtls_write_records
does - so we convert it to use those functions instead.

Reviewed-by: Richard Levitte <levitte@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Hugo Landau <hlandau@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/19424)