Make open/write defer opening an object-stream (PUT), like open/read does (with GET)

[jti-lanl]

This was item "(3)" in issue #83. Moving here, so we can close that.

This comes up for the MD scaling demo. Otherwise, we don't particularly care about zero-length files.

Refactor MarFS write() to work like read()
-- don't start the PUT in open(), wait until the write().
-- zero-length files might have no object? (Maybe that's good.)
-- tweak code to handle zero-length files?

The point of this is to make it easier/cheaper to write zero-length files. Currently, we'd pay the cost of opening/closing a PUT for each zero-length file.

But, zero-length files invite questions, which I've recorded in item #111.

[jti-lanl]

The suggestion in #111 to use a DIRECT repo, for size=0, would make this problem go away (?)

[brettkettering]

Related to #96.

[brettkettering]

A good candidate for the DAL (Data Abstraction Layer) work.

[wfvining]

Jeff and I decided we should do this in MarFS itself, rather than leaving it to the various DAL implementations. I have a working implementation in the lazy-open branch which needs further integration testing against both the OBJ and MC DALs, but should be just about ready to merge.

[wfvining]

This has been done as part of implementing marfs_flush().