Add Read only mode on jsonDB

[gaelL]

Simply add a readonly arg to jsonDB to not try to lock.

For example the ls url just need to read. No need to lock the db for that. https://github.com/guits/pastefile/blob/master/pastefile/app.py#L207

We also don't want to return a return "Lock timed out" in that case. Only read only database is enough.

[gaelL]

It will reduce the risk of having concurrent lock

[gaelL]

Checking the code, in fact if we don't want to lock and use the jsonDB as a read only,
just don't use the with statement.

db = jsonDB()
db.load()

Will return the db without lock. Just take care to not save but for feature like ls it's enough

[gaelL]

TODO rewrite part off controller.py to not lock the db.