[BUG] Storage quota value drift

[benbierens]

Testing with revision 5597afa - Feat/overlay support (#94) reveals that quota values drift as data is stored and then cleaned up in the node.

Test description:

• Upload a file of known size
• Check that /data reports the new file
• Check that /space reports correct quota-bytes-used
• Wait until cleanup
• Check that /data reports no files
• Check that /space reports 0 quota-bytes-used
• Repeat these steps n times

After each loop:

• The file (manifest) is correctly removes from /data.
• The blocks and manifest files are correctly removed from the data-dir.
• /space incorrectly reports a single block still being stored. Number of leftover blocks increases by 1 every loop.
• /space incorrectly reports some number of quota-bytes-used instead of zero. Number of leftover bytes increases by a constant amount every loop. The amount it increases by seem to be somehow related to the filesize of the uploaded file. It's usually around 126 bytes.

I'm pretty sure the quota numbers are currently being used by the marketplace logic to decide whether to pick up slots. Having these numbers drift will therefore eventually lead to trouble. Additionally, external applications monitor their nodes using /space, so it'll be a problem there too.