Clean pending_snapshot directory of aggregator

[jpraynaud]

Issue

The pending_snapshot directory is never cleaned and some uncompleted snapshots stay forever.
This can create a storage issue (particularly with the mainnet archives).

To do

• Clear pending_snapshot before or after snapshot archive is created
• Clear pending_snapshot at aggregator startup

[dlachaume]

The temp files in the pending_snapshot directory were only removed after a successful upload.

Now, pending_snapshot directory is cleaned at aggregator startup and uncompleted snapshots archives are removed at every step :

• if an error occurs during the creation of the archive
• if an error occurs during the upload

More details in the PR #1011 description.