Read blocks into off-heap if caching is disabled for read

Currently we decide whether a disk read shouldUseHeap based on three criteria:

1. If block cache is disabled, return false
2. If block cache is anything other than CombinedBlockCache, return true
3. Otherwise return false for DATA blocks and true for other blocks

The assumption here is we're making the decision based on which cache the block is likely to end up in. But if the read has caching disabled (i.e. setCacheBlocks(false)), it won't get into any cache. So we should return false in that case too.

The only caller to shouldUseHeap in HFileReaderImpl has a boolean cacheBlock available which determines if the read block should attempt to be cached. We can pass that boolean into the function. We should probably also account for cacheConf.shouldCacheBlockOnRead for the same reason.