Bugfix #340: remove mmap buffer, require msgpack==0.5.0

[jamesmunro]

• remove mmap buffer
• require msgpack==0.5.0 (we need strict_types)
• consolidate msgpack usage into _msgpack_compact
• add metadata size warning at 8MB (max is 16MB)

Reference Issues/PRs

Fixes #340

What does this implement or fix?

We've been allocating a 4GB buffer for every data pack operation (not metadata).
There are a few issues with this,

• It's a very large overallocation in lots of cases, so inefficient
• It can fail even for small data, this was most common on windows builds

This change also improves some other tightly coupled aspects of out msgpacking.

• Consolidating msgpack usage into _msgpack_compact
• Creates padded (mod 8 bytes) version of packb so that we can store in uint64 column without extra copies, and removes bugs with original padding approach.
• Fixes ambiguity between list and tuple storage and require strict_types. Previously we where relying on calls to packb both when strict_types argument wasn't available, and when the strict_types check failed, they where both caught by TypeError.
• Stop throwing ArcticDbNotYetImplemented when packed data is over 4GB, and instead let MemoryError propagate from msgpack when it actually hits the limit (which is similar).
• The 16MB limit on metadata will give users a sudden hard to recover error when they hit it, so introduce a warning level at 8MB, to encourage users away from storing any more in metadata.

Checklist

Checklist for code changes...

• Have you updated the relevant docstrings, documentation and copyright notice?
• Is this contribution tested against all ArcticDB's features?
• Do all exceptions introduced raise appropriate error messages?
• Are API changes highlighted in the PR description?
• Is the PR labelled as enhancement or bug so it appears in autogenerated release notes?