Re-enabled on-disk collapsing of entries after fixing related code.

Re-enabled on-disk collapsing of entries after fixing related code.

Since we started writing partial entries, we cannot rely on negative sidNext
marking the end of the slice/write sequence. Added a WriteRequest::eof field
to signal that end explicitly.

Do not leak db slices when write fails or IoState is closed before the write
succeeds.

Handle store client requesting an offset we have not stored yet. This might
happen for collapsed hits (and also if the client is buggy). May need more
work to slow the reader down.

Do not update various shared stats until the corresponding slot is written.