Two metadata cleanups:

1. Change metadata to keep an open cursor handle and spinlock around it for single update/insert/remove operations (see the lookaside table code for an example), instead of re-opening a new metadata cursor on each access.

Open question: should we continue to keep the meta dhandle cached to improve performance of operations where we are opening a new metadata cursor (for example, btree-apply, or hot-backup, where we're traversing the entire metadata file. I'm guessing the answer is yes, we already have the code and it's not worth removing.

2. Change code references from "metadata file" to "metadata table".