Add Volume atomic multi-slot appends.

[tonyastolfi]

• Adds llfs::Volume::MultiAppend, for atomic appending of multiple slots
• Fixes a bug in Volume's append job, where an error status returned from preparing the job could (incorrectly) fail a check (generating a panic)
• Adds support for block-device level simulation of LogDevice in StorageSimulation, based on IoRingLogDevice
• Fixes bug where moving an IoRing object after using it to construct an IoRing::File would result in a dangling pointer
• Minor enhancements to various APIs:
  • variable-length integer pack/unpack now accept ConstBuffer/MutableBuffer
  • DataReader::read_token added
  • separate halt() and join() member functions for LogDevice, instead of the combined close() function (now deprecated)
  • relax the restriction that LogDevice::prepare may only be called once per commit (now you can call it multiple times to progressively increase the size of the prepared buffer, provided there is space in the log)
• Consolidated various unit test configuration parameters in llfs::testing::TestConfig (llfs/testing/test_config.hpp)
• Renamed llfs::PageArena::close to halt to be more consistent with other parts of the code
• Added new diagnostics to StorageSimulation when a possible deadlock is detected (main test function has exited and there is no runnable work, but the work count on the simulation execution context is non-zero)
• Fixed a bug in static/runtime configuration of (ioring) log devices where it was possible to create more flush ops than the number of blocks in total, resulting in race conditions where multiple flush ops would write data to the same location(s)

[gabrielbornstein]

Just got started at the end of day today. I'll have more comments, but here's my first couple.

[gabrielbornstein]

A couple of more comments I made yesterday and forgot to publish.

[gabrielbornstein]

Done with feedback! Let me know your questions.