Use common tls_write_records() even for DTLS

Use common tls_write_records() even for DTLS

In practice this just means have a DTLS specific write_records that the
common tls_write_records() just calls. We also replace the use of
ssl3_write_pending() with tls_retry_write_records().

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)