This is a parameter to the getdns_context that tells the context how
much to pad queries that go out over TLS.
It is not yet functional in this commit, but the idea is to pad each
outbound query over TLS to a multiple of the requested blocksize.
Because we only have a set amount of pre-allocated space for dynamic
options (MAXIMUM_UPSTREAM_OPTION_SPACE), we limit the maximum
padding blocksize.
This is a simplistic padding policy. Suggestions for improved padding
policies are welcome!