On python3.6 this raises:

 sqlite3.OperationalError: cannot VACUUM from within a transaction

  vacuum as little as possible, as it is a heavy operation to perform.

  To allow more concurrent access to the sqlite

  This normalizes the result on python2 and python3, and as well, makes the
  result more accurate.

  This may case more memory usage and more 'live' data to handle,
  for this reason, I'm making it configurable

  This is just to clean the methods a bit

/reviewed-on nexedi/slapos.core!147

Currently it is just sqlite3.connect(), but we'll want to add connection
autotuning in the next patch, so before that let's consolidate all
sqlite opening places in the code into one.

Partition folder snapshot use du to calculate folder size and return value in Mb.
This snapshot is enabled by default and will calculate size of each partition every 24hours.
Partition with size < 1Mb are not are not reported (considered as empty).
The behavior can be changed using some configurations in slapos.cfg:

[collect]
report_disk_usage = True

disk_snapshot_process_pid_foder = /srv/slapgrid/var/run

disk_snapshot_time_cycle = 86400

 In relation to data collection this commit introduces:

  - data collector for temperature (parsing sensors)
  - Heating testing for determinate the Zero Emission Ratio of the computer.

 In relation to reports this commits introduces:

  - Consumption report for computer usage (CPU Load, Zero Emission Ratio, Memory)
  - Consumption report for partitions (CPU Load and Memory)
  - All consumption reports uses TioSafe XML Format

  Remove older registers, already reported (or exported to csv)
  (older them 3 days)

Major changes
  Rename pymonitor, add dependency and simplify
  Save all information using sqllite
  Record information from all processes individually
  Implement Snapshot for Computer and Systems information
  General Clean up

  Remove older registers, already reported (or exported to csv)
  (older them 3 days)

Major changes
  Rename pymonitor, add dependency and simplify
  Save all information using sqllite
  Record information from all processes individually
  Implement Snapshot for Computer and Systems information
  General Clean up