In the olden days, we passed it a vector of open ESMReader instances, as they knew the filenames and sizes, so were a convenient source of this knowledge.
When the ReadersCache was introduced as a pool of readers to limit the maximum number of simultaneously open file handles (to avoid going over the OS' limit) it was a poor substitute.
* We iterate over all the earlier readers in order in a double loop, which is the worst case scenario for an LRU pool as once we're past the size limit, we're guaranteed maximum thrashing - the least recently used item is the most likely to be used next, so the worst to evict.
* We didn't want to read any ESM files, just know whether they'd been read and what their sizes were, so didn't want to open a file handle, which the ReadersCache forced us to do.
Obviously, opening lots of file handles isn't fast, and as this was an operation done for each content file which iterated over the file's masters and within that loop iterated over every loaded file, that's O(n^3) complexity in the worst case, and for things like delta plugin merged plugins, they hit the worst case in long load orders.
This resolves the freeze reported as https://gitlab.com/OpenMW/openmw/-/issues/8425, but there may be other freezes on launch.
Use LRU cache for ESMReaders. When cache capacity is reached close least
recently used ESMReader. Remember the file name if a reader was open. Once the
reader requested again open the file if there is stored name for it. Put
released ESMReader to the back of the free items list. Close ESMReader's from
the front of the free items list.
Cached item can be used only by one client at the same time. If the same item is
requested twice exception is thrown. This should never happen in practice. If
this happens need to fix the client logic.
It's allowed to go over the capacity limit when requesting different readers.
Ideally this should never happen but there will be system error anyway
signalizing about too many open files. Need to fix client logic in this case.
All places that were using a vector of ESMReaders now using the cache. Cache is
local for each use case and there is no need for a thread safety.
