implement mark and sweep GC

License: MIT
Signed-off-by: Jeromy <jeromyj@gmail.com>

dont GC blocks used by pinner

License: MIT
Signed-off-by: Jeromy <jeromyj@gmail.com>

comment GC algo

License: MIT
Signed-off-by: Jeromy <jeromyj@gmail.com>

add lock to blockstore to prevent GC from eating wanted blocks

License: MIT
Signed-off-by: Jeromy <jeromyj@gmail.com>

improve FetchGraph

License: MIT
Signed-off-by: Jeromy <jeromyj@gmail.com>

separate interfaces for blockstore and GCBlockstore

License: MIT
Signed-off-by: Jeromy <jeromyj@gmail.com>

reintroduce indirect pinning, add enumerateChildren dag method

License: MIT
Signed-off-by: Jeromy <jeromyj@gmail.com>