Wrap repository writes into our storage format file.

[poolpOrg]

/!\ immutable
/!\ non-rewindable

STORAGE FILE FORMAT:
    [uint32] Version
    [uint32] Data type
    [blob]
    [32 bytes = checksum(raw blob)]
    [32 bytes = checksum(full)]
    [64 bytes = signature(checksum(full)) || nul-bytes]


PACKFILE:
    [uint32] Version of packfile
    [uint32] Data type
    [blob] <----- serialized file
    [32 bytes = checksum(raw blob)]
    [32 bytes = checksum(full)]
    [64 bytes = signature(checksum(full)) || nul-bytes]


STATE:
    [uint32] Version of packfile
    [uint32] Data type
    [blob] <----- serialized state
    [32 bytes = checksum(raw blob)]
    [32 bytes = checksum(full)]
    [64 bytes = signature(checksum(full)) || nul-bytes]

[poolpOrg]

format has been reworked to:

/!\ immutable
/!\ non-rewindable

STORAGE FILE FORMAT:
    [uint32] Data type
    [uint32] Version
    [blob]
    [32 bytes = HMAC(all of the above), masterKey]

It is part of the #425 PR