Minimal crash recovery for the blobs network transfer manager

Minimal set:

• * Verification failures #8825 (closed)
• * Retry strategy #8822 (closed)
• * deferred for full sync completion #8848 (closed) (would help 8822 as we could also retry the traditional sync)
• * extra care with concurrent download/upload #8932 (closed)
• * sync state on client side #8823 (closed)