The HTTP cache for the API now takes a parameter that determines if it

is created in memory (create_db_in_memory). This parameter replaces

the db_path parameter, which previously accepted the special value

':memory:' to create the SQLite database in memory. The new

parameter is more straightforward because caches will only ever be

required to be created at the default location (for normal operation)

or in memory (for unit testing).
The default API cache path is stored on the APICache class as the

CACHE_PATH constant, which can be used without instantiating the

class. Becuase the HTTP cache for the API is only ever stored in one

location on disk, it can be cleared (i.e., unlinked) without

instantiating the class to determine where it is located on disk. This

is important in the case where the cache exists but has become corrupt

and cannot be loaded (and so instantiation of the APICache class will

fail).

Unit tests pass.