Create client module from bclient

[braydonf]

Closes #834

[tynes]

NACK until there is a clear upgrade path for users of just bclient itself. @BluSyn thoughts?

This change will blow up the size of the bundle for anybody that wants just the client since bcoin doesn't support es modules (with tree shaking) yet.

[braydonf]

@tynes This adds the client to bcoin so that it can move with tightly coupled parts within wallet. I would imagine other applications, similar to wallet, that depend on bclient would have a similar need and would do something similar. However, this is not removing bclient, which can still continue to be used, as well as bcurl.

[tynes]

ACK on the agreement that any changes to the bclient codebase that is brought into bcoin will be done in a way that makes it easy to backport the changes to the bclient package. This will prevent users of bclient from falling out of compatibility.

Its likely there won't be a ton of changes over time, so this should be manageable to do.