Lowered the default value of CLUSTER_CACHE_SIZE

!!! IMPORTANT !!!

For this change to take effect when rebuilding/recompiling the project in an
existing build directory, the following command must be explicitly
executed:

```
meson configure -D CLUSTER_CACHE_SIZE=16777216
```

---

The default value of CLUSTER_CACHE_SIZE was made low-end-device-friendly
- the cluster cache max memory size was lowered from 512MiB to 16MiB.
For mono-ZIM-file use cases (when a single ZIM-file is open at a time)
this approximately returns the memory usage by the cluster cache to
how it used to be before the cluster cache was made global (shared
across all open ZIM-files).

Note however that there is some ambiguity with respect to what memory
usage means - there is virtual memory vs physical memory. Current
implementation of cluster cache memory management targets physical
memory usage on systems supporting mmap, where allocation of virtual
memory by the cluster cache can exceed the actual physical memory usage
by a factor of ~5x-9x.

On systems not supporting mmap (or with an implementation of malloc not
utilizing mmap for large allocations) the consumption of physical memory
should equal the size of allocated virtual memory, and the actual memory
usage will exceed the cache memory limit by the said factor, i.e. as
much as 144MiB can be consumed by the cluster cache at the default
setting.

Author: veloman-yunkan

meson_options.txt

@@ -1,5 +1,5 @@
-option('CLUSTER_CACHE_SIZE', type : 'integer', min: 0, max: 1000000000000, value : 536870912,
-  description : 'set default cluster cache size in bytes (default:512MB)')
+option('CLUSTER_CACHE_SIZE', type : 'integer', min: 0, max: 1000000000000, value : 16777216,
+  description : 'set default cluster cache size in bytes (default:16MiB)')
 option('DIRENT_CACHE_SIZE', type : 'string', value : '512',
   description : 'set dirent cache size to number (default:512)')
 option('DIRENT_LOOKUP_CACHE_SIZE', type : 'string', value : '1024',