Instead of having DiskBackedFS allocate a ByteBuffer, leave it to each
client to provide buffer space.
This is significantly faster in many cases where we can use a stack
buffer and avoid heap allocation entirely.
The hashmap cache was ridiculously slow and hurt us more than it helped
us. This patch replaces it with a flat memory cache that keeps up to
10'000 blocks in cache with a simple dirty bit.
The syncd task will wake up periodically and call flush_writes() on all
file systems, which now causes us to traverse the cache and write all
dirty blocks to disk.
There's a ton of room for improvement here, but this itself is already
drastically better when doing repeated GCC invocations.
This way you can spam small write()s on a file without the kernel writing
to disk every single time. Flushes are included in the FS::sync() operation
and will get triggered regularly by syncd. :^)
