server trade suspension

[chappjc]

The server's handling of trade suspension needs to be fleshed. The spec currently describes a single epoch returned by the server at which the markets will no longer be available. However, each market has its own epoch duration, so there should be a suspension epoch returned for each of the markets. A spec up date will also be needed.

This issue refers to the server's suspend/resume capabilities (e.g. stop new orders, set a final epoch for each market, wait for swaps to complete with a time limit, adding an admin interface suspend command with target time, and all the resume functionality including jump starting active swaps.)

#269