Rework the data path and db type handling.

This commit modifies the way the data paths are handled.  Since there will
ultimately be more data associated with each network than just the block
database, the data path has been modified to be "namespaced" based on the
network.  This allows all data associated with a specific network to
simply use the data path without having to worry about conflicts with data
from other networks.

In addition, this commit renames the block database to "blocks" plus a
suffix which denotes the database type.  This prevents issues that would
otherwise arise if the user decides to use a different database type and
a file/folder with the same name already eixsts but is of the old database
type.  For most users this won't matter, but it does provide nice
properties for testing and development as well since it makes it easy to
go back and forth between database types.

This commit also includes code to upgrade the old database paths to the
new ones so the change is seamless for the user.

Finally, bump the version to 0.2.0.