cbbackup-incremental reimplements tap handling

The backup tool has its own tap implementation rather than using the one that already ships with the product.

The attached patches do two things:

1. modularize the code a bit so that the inner loop of the tap handling is a separate piece of code.
2. use existing tap implementation

The first patch is just a refactoring:


 management/mbbackup-incremental | 292 +++++++++++++++++++++------------------
 1 files changed, 155 insertions(+), 137 deletions(-)


The second is an overall code reduction by getting rid of stuff we already had in our tap lib:


 management/mbbackup-incremental | 113 +++++++++++----------------------------
 management/memcacheConstants.py | 2 +
 2 files changed, 34 insertions(+), 81 deletions(-)


This is built on top of the asynchronous data storage processing patch from MB-4573.