PTFM-12403 Tab completion fixes

- Update argcomplete dependency to pull in a copy that manages spaces after individual completions
- Remove space escaping logic that is no longer necessary (since argcomplete correctly escapes completions)
- Clean up a bit of dead code

This change also highlights the fact that the upload command in dx sh lacks local path completions and option completions. Code in DXCLICompleter (used by dx sh) is redundant with the completion hook logic in completers attached to the argparse parser, and with the completion driver logic in argcomplete. Work on eliminating this redundancy is blocked on https://github.com/kislyuk/argcomplete/issues/83 for now.
Enable remaining uses of LocalCompleter
Fix local path completer for dx build
PTFM-12427 Omit info message in dx login --token
Parameterize and reduce checksum read block size

Read block size reduced from 100M to 4M. Empirically determined to be faster, probably due to speed issues with large memory allocations.
Java: allow retries when server is unavailable (if route permits it)
Java: raise number of retries from 5 to 6.
PTFM-12309 Adding call to JSON::get<std::string>()

This fixes an LLVM/Clang build error in the upload agent.
PTFM-12309 One last change to squash LLVM errors

This patch makes it possible to build api_helper.cpp with both LLVM and
GCC; before this LLVM/Clang would error out on these lines with "error: use of
overloaded operator '==' is ambiguous".
PTFM-12309 Added README for OS X 10.9 build setup
Changelog and version updates for v0.106.0