Review DTLS resources cardinality

[ghjansen]

This issue is related to #257.

Currently, the handshake process performed by BoucyCastle allows to use multiple certificate resources, multiple signature algorithms and only one key resource. Such cardinality doesn't seems to fit comfortably with the configuration format proposed on #257, so it would be good to investigate further and adapt the configuration and/or implementation accordingly.

Notes:

• If certificate resources refers to public key and we intend to allow the configuration of multiple resources, that could be done through a separation token like ; e.g.

<certificate path="../certificate1.pem; ../certificate2.pem;" key="../key.pem" algorithm="rsa"/>

or maybe using a different structure like

<certificate key="../key.pem" algorithm="rsa">
    <resource path="../certificate1.pem"/>
    <resource path="../certificate2.pem"/>
</certificate>

• For multiple signature algorithms the format suggested in #257 properly groups the resources in different elements

<certificates>
    <certificate path="../certificate.pem" key="../key.pem" algorithm="rsa"/>
    <certificate path="../certificate.pem" key="../key.pem" algorithm="ecdsa"/>
</certificates>

but apparently only one key resource resource is allowed per handshake, what doesn't allow to use more than one <certificate> per attempt.