Revert "I'm feeling brave"

This reverts commit dd54e49d6a6cac73631efb9b17f4e2e07c63bd34, which only
happened to work sometimes not really.
Rewrite SQLiteConnectionPool

Now the pool itself handles queuing operations, based on
round-robin'ing a fixed number of connections
Fix all the operations

This also makes Reset properly signal completion
PersistentBlobCache should only know about AsyncConnection
Merge pull request #66 from github/fix-sqlite3-use

Fix parallel usage of Akavache SQLite3 connections