Move sequence increment to post encryption processing

This change make dtls_write_records virtuall the same as
tls_write_records_default, which will enable us to merge them in a
subsequent commit.

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)
