Update oslo.db from oslo-incubator

This patch contains:

1 - changes from oslo-incubator to commit
    af5f71096c7bb83fe2f956588a9ec643b0d9a2f6
2 - Patch ContextFormatter for imparting context info
    Gerrit change-id Ibe6edf1fcf111481abfd750fcbef89209ee9d615
3 - Patch Drop dependency on log from oslo db code
    Gerrit change-id I6ec426274eb3b603dbbc894762edb97515a618b2