Add cache for collections

[mmacata]

With this PR, the response to /collections is cached directly in the openeo-grassgis-driver without the need for nginx or similar to handle the cache.
The cache will be filled on application startup and can be cleared with /collections?cache=false

Related to #85

[metzm]

It seems that the cache is not filled at application startup, but when the endpoint /collections is used for the first time after startup. Is this correct? In this case it would be nice if the collections cache would be already filled on application startup.

[mmacata]

Yes this is correct. I wanted to avoid a long startup time, because processes are fetched on startup as well. When it takes too long the application never starts but runs in a timeout. The way it is now, the whole application would not be affected and there is more time for collections to be collected. I would prefer to keep it this way and maybe add a persistent file based or redis cache in the future which survives a restart of the application.

[metzm]

[...] I would prefer to keep it this way and maybe add a persistent file based or redis cache in the future which survives a restart of the application.

Very good idea, makes sense.

[metzm]

Ready for merging IMHO.