multi: Add tpl update reason to work ntfns.

[davecgh]

This updates the new RPC websocket work notification that is sent when a new template is generated and a websocket client requests updated via notifywork to include the reason the template was generated.

The text-based reasons delivered via the work notification are:

• newparent -- The template builds on a new block as compared to the previous one.
• newvotes -- A new vote for the block the template builds on has been received.
• newtxns -- New non-vote transactions are available and have potentially been included.

This allows pools that subscribe for work notifications to better choose when to signal workers to throw away all current work and start working on a new template immediately. In particular, the newparent and newvotes reasons should trigger workers to immediately switch. On the other hand new work as the result of newtxns is something that pools will likely want to allow workers to opportunistically switch to after completing a a work cycle.

It is also important to note that miners should roll time timestamps as they work on templates in between updates to keep the timestamps as close to accurate as possible.

This consists of two commits to make the review easier.

The first commit exports the template update reason type and associated constants in preparation to modify the template notifications to include the reason a new block template was generated.

The second commit modifies the background template generator subscription notifications, the RPC websockets notification manager, and the JSON-RPC work notification to include the reason and updates all of the intermediate plumbing code accordingly.

It also updates the JSON-RPC documentation to include the new field and enumerate the supported reasons.

[davecgh]

For reference, in addition to doing a significant amount of corner case testing with simnet before submitting this PR, I let it run over the weekend on mainnet while logging the results, . There were only two cases where the initial newparent work notification had fewer than maximum cast votes (meaning the timeout provided for votes to arrive is set reasonably) and both cases were followed up with a newvotes work notification that included all votes as soon as they arrived.