Feature limit caching to prescribed number of bytes per file (#6009)

* feature: we can check if a fileId is already in the cache

We using this to protect cache from adding the same needle to
the cache over and over.

* fuse mount: Do not start dowloader if needle is already in the cache

* added maxFilePartSizeInCache property to ChunkCache

If file very large only first maxFilePartSizeInCache bytes
are going to be put to the cache (subject to the needle size
constrains).

* feature: for large files put in cache no more than prescribed number of bytes

Before this patch only the first needle of a large file was intended for
caching. This patch uses maximum prescribed amount of bytes to be put in
cache. This allows to bypass default 2MB maximum for a file part stored
in the cache.

* added dummy mock methods to satisfy interfaces of ChunkCache

weed/filer/reader_at_test.go

@@ -31,6 +31,14 @@ func (m *mockChunkCache) ReadChunkAt(data []byte, fileId string, offset uint64)
 func (m *mockChunkCache) SetChunk(fileId string, data []byte) {
 }
 
+func (m *mockChunkCache) GetMaxFilePartSizeInCache() (uint64) {
+	return 0
+}
+
+func (m *mockChunkCache) IsInCache(fileId string, lockNeeded bool) (answer bool) {
+	return false
+}
+
 func TestReaderAt(t *testing.T) {
 
 	visibles := NewIntervalList[*VisibleInterval]()