Store BGPsec certs info, print at configured output

Store BGPsec certs info, print at configured output

-Use struct 'router_key' to represent BGPsec certificates.
-Use reference count of Subject Key data (ID and public key) with 'sk_info' struct.
-Add router certificates info to local DB, as part of the refactor 'roa_table' was renamed to 'db_table'.
-Add configuration parameter 'output.bgpsec', the information is printed in hexadecimal representation (there's a TODO here: maybe encode and print as base64).
-Consider BGPsec keys in deltas.
-Use a constant for SKI length, since always is the same (20 bytes/octets).