limit the size of buffer to not do not exceed memory size
Removed unnecessary scripts
Refactor backends and metrics logic

* Divided colmet package into three parts
- colmet-node : Retrieve data from taskstats and procstats and send to
collectors with ZeroMQ
- colmet-collector-hdf5 : A collector that stores data received by
ZeroMQ in a hdf5 file
- colmet-common : Common colmet code
* Added some parameters of ZeroMQ to prevent a memory overflow
* Simplified the command line interface
Added new console_scripts entries : colmet-node and colmet-hdf-collector
update changelog