`Store.syncSegments()` must sync root segment *last*

[clarisma]

Important at the end of a transaction:

• If segments are synced in arbitrary order, this could create a race condition (index entries visible before tile data)
• does not affect consumers blocking on a request (signaled once transaction finished), or processes waiting on lock (only released at tx end)
• But another process checking the index may see that a tile is presented, when in reality its data is still being written!

[clarisma]

Must write index last, not just root segment.

[clarisma]

Could establish explicit write order for blocks. Maybe: markSequential(long pos) with auto-increment count. Blocks are written in count order. Blocks with count 0 can be written in arbitrary order.