Teach catmaid_fetch to cache some return values

[jefferis]

Ideal arrangement would be that

• you could choose which calls are cached
• you could choose different cache time outs

(see also #20 for which this might be useful)

[jefferis]

This would be useful for e.g. catmaid_get_annotationlist which is starting to get expensive (>1.5s).

[jefferis]

It would be better to do this after merging the http::VERB to simplify this section

https://github.com/jefferis/rcatmaid/blob/bfb757f73561a12aec39da2be77b457d42a694ab/R/catmaid_connection.R#L364-L373

In particular it is essential that the httr::config is part of the memorisation arguments.