1. Case1: Fuse server exits in thread not main. There is possibility
       that process finishes before shutdown of server.
    2. Case2: Fuse server exits in thread of state machine. There is
       possibiltiy that state machine not responses to signal catch
       thread. Then dead lock happens. Process exits before shutdown of
       server.

    This pr aims to seperator shutdown actions from signal catch
    handler. It only notifies controller. Controller exits with
    shutdown of fuse server. No race. No deadlock.

Signed-off-by: 泰友 <[email protected]>

    There are scenes that cache file is smaller than expect size. Such as:

    1. Nydusd 1.6 generates cache file by prefetch, which is smaller than size in boot.
    2. Nydusd 2.2 generates cache file by prefetch, when image not provide ext blob tables.
    3. Nydusd not have enough time to fill cache for blob.

    Equality check for size is too much strict for both 1.6
    compatibility and 2.2 concurrency. This pr ensures blob size smaller
    or equal than expect size. It also truncates blob cache when smaller
    than expect size.

Signed-off-by: 泰友 <[email protected]>

    Nydus2.2 has same format for blob cache and bitmap on disk. However,
    2.2 nydusd not use blob cache of 1.6 nydusd. This pr counts ready
    chunk number for both 1.6 and 2.2, instead of thinking it as not filled
    at all.

Signed-off-by: 泰友 <[email protected]>

    User io may involve discontinuous segments in different chunks. Bad
    read is produced by merging them into continuous one. That is what
    Region does. This pr separate discontinuous segments into different
    regions, avoiding merging forcibly.

Signed-off-by: 泰友 <[email protected]>