Revert "Instantiate database object at top-level"

This reverts commit 01dab376b012283bcd53a5449a3669d9a15cc7b3.

Conflicts:
	src/github.com/otoolep/rqlite/server/server.go
More diagnostic output
Lock sqlite database during changes

SQLite can return BUSY if 2 or more threads attempt to change it simulataneously