Every segment file stays open for the life of the engine #68

PageStore opens every segment file at startup and keeps a FileChannel for each until the engine closes. So the number of open files grows with the size of the database: about 400 for 100 GB at today's 256 MiB segments, and thousands if segments get smaller (#52).

The engine should manage this itself:

• keep at most N segment files open, as a new setting
• open a file when a page in it is needed, and close the least recently used one when the pool is full
• never close a file another thread is using
• never close a file with unsynced writes before forcing it, since sync commits rely on those pages being on disk
• sync() should only force files written since the last sync, not every file

Also try memory-mapping sealed segments (read-only, with the file handle closed after mapping) next to the pool. Keep it only if it can be made reliable: a mapped file that is truncated or deleted underneath can crash the process instead of raising an error, and mapped memory is harder to bound.

Needed before #52 can make segments smaller.