Rethink the transient cache in Request Engine

[hrj]

The request engine maintains two types of caches: a transient (in memory) cache and a persistent (on disk) cache for cacheable responses.

The purpose of the transient cache is not clear to me. I think the disk cache should be sufficient as most OSes will cache the disk i/o anyway.

@oswetto seeking your help answering this one, in case you have figured it out in your fork of lobobrowser

[hrj]

There is a benefit in having a transient cache if the persistent cache is encrypted; the transient cache will help reduce the overhead of decryption for often used responses.

Note that the persistent cache is not currently encrypted, but it will be desirable to do so in the future.

Hence, it is better to keep the transient cache.

We can, however, redesign the cache system so that the distinction between transient and persistent caches is an internal implementation detail of the cache, rather than spreading the code throughout the request engine.