ripple/rippled

The new 'Domain' field allows validator operators to associate a domain
name with their manifest in a transparent and independently verifiable
fashion.

It is important to point out that while this system can cryptographically
prove that a particular validator claims to be associated with a domain
it does *NOT* prove that the validator is, actually, associated with that
domain.

Domain owners will have to cryptographically attest to operating particular
validators that claim to be associated with that domain. One option for
doing so would be by making available a file over HTTPS under the domain
being claimed, which is verified separately (e.g. by ensuring that the
certificate used to serve the file matches the domain being claimed) and
which contains the long-term master public keys of validator(s) associated
with that domain.

Credit for an early prototype of this idea goes to GitHub user @cryptobrad
who introduced a PR that would allow a validator list publisher to attest
that a particular validator was associated with a domain. The idea may be
worth revisiting as a way of verifying the domain name claimed by the
validator's operator.

Resource limits were not properly applied to connections with
known IP addresses but no corresponding users.

Add unit tests for unlimited vs. limited ports.

An audit showed that a number of the RPC error codes in
ErrorCodes.h are no longer used in the code base.  The unused
codes were removed from the file along with their support code
in ErrorCodes.cpp.

The ledger already declared a transaction that is both single-
and multi-signing malformed.  This just adds some checking in
the signing RPC commands (like submit and sign_for) which allows
that sort of error to be identified a bit closer to the user.

In the process of adding this code a bug was found in the
RPCCall unit test.  That bug is fixed as well.

* The functions removed in this commit were explicitly defaulted
  but implicitly deleted

* docker container definitions for package building
* cmake targets for building packages
* initial gitlab CI + artifactory integration

* Use TLS 1.2
* Make certificate verification configurable

Trailing comments are not permitted in the crawl section
and can cause the lines containing them to be ignored.

If the number of peers a server has is below the configured
minimum peer limit, this commit will properly transition the
server's state to "disconnected".

The default limit for the minimum number of peers required was
0 meaning that a server that was connected but lost all its
peers would never transition to disconnected, since it could
never drop below zero peers.

This commit redefines the default minimum number of peers to 1
and produces a warning if the server is configured in a way
that will prevent it from ever achieving sufficient connectivity.