Add pt() method to eventlet backdoor

pt() will print stack traces from all real Threads.
Patch logging to work with eventlet's tpool

threading locks do not work with eventlet's tpool due to some bug.
This adds some lock routines which are implemented as spin locks in
a way that bypasses eventlet issues.
MYSQLDB: Add framework for mysqldb impl

This re-works the db_backend importing to better support multiple
DB API implementations, where new implementations may use a class
to define their methods.  Also allows importing db implementations
by full path that are unknown to nova, so you don't have to
modify db/api.py for new implementations.

Implements bp db-mysqldb
MYSQLDB: Add insert() to mysqldb/connection

Adds generic insert() call to mysqldb connection that takes a table
name and a dictionary of column/values to insert.

Implements bp/db-mysqldb
MYSQLDB: Add bw_usage_update

Adds implementation for bw_usage_update

Implements bp/db-mysqldb
added bw.py