Synapse API initial version

This release includes:

• "synapse" datatype without label support for now
• /api/server/compiled-types endpoint that returns all compiled datatypes for this DVID instance
• various bug fixes and minor enhancements

Multi-database version of DVID

DVID serve command now requires specification of TOML configuration file that allows specification of different mutable, immutable, and metadata storage engines. Currently, only the imagetile datatype uses the immutable store. In the future, all committed versioned data will eventually be moved (or stored directly) into the immutable store if one is specified.

The DVID github wiki has been revised to reflect the new installation and run system.