[Settings] Cache settings after first retrieval

[driusan]

There are a number of settings that we use frequently in LORIS such as "base" to get the base path. When accessed multiple times in a request at different places in LORIS, each one currently requires a query against the database.

This caches any settings that are used in NDB_Config so that future calls to getSetting() can return the cached value and not make another round trip to the DB.

[skarya22]

Testing shows that this did not introduce any issues in modules that use the getSetting function, however I wanted to note that it seems that although it has reduced the number of DB pulls, it seems that sometimes it skips over the cache for some reason.
I set the function to log when it pulls from cache, and when it pulls from DB. If you look at the useEDC field you can see here that it pulls it three times when I load candidate_list, it pulls from the DB twice, and uses the cached value once. Not necessarily an issue, but just something to note incase we want to improve it further

[driusan]

@skarya22 is it possible the logs are coming from different requests?

[skarya22]

@driusan I suppose it is possible, but shouldn't it still ideally pull from the same cache?

[driusan]

No.

I mean, ideally, yes. But I don't think that is possible with PHP (at least not in any easy way) since each request is isolated in its own process/thread with its own state.