[unsharding] Follow up on some low-priority fixes/renames/refactors

[schomatis]

This issue tracks small things that need to be done to wrap up the unsharding effort (concentrated in #94) but should be taken care of at the end as they are not very complex nor essential, and addressing them now might disrupt the current stack of PRs in development/review without adding much value.

Working in branch https://github.com/ipfs/go-unixfs/tree/schomatis/directory/unsharding-minor-issues.

• Document asynchronous choice (Size-based unsharding #94 (comment)).
• Change UpgradeableDirectory name (Size-based unsharding #94 (comment)).
• Un-export all Basic/HAMT switch functions (Size-based unsharding #94 (comment)).
• Make return values explicit in complex functions (Size-based unsharding #94 (comment), Size-based unsharding #94 (comment)).
• Rename inner HAMT functions to swap/take accordingly (Size-based unsharding #94 (comment), Size-based unsharding #94 (comment), Size-based unsharding #94 (comment)).
• Settle on a default number for HAMTShardingSize (Size-based unsharding #94 (comment)).
• Clean up getDagService() member function (Size-based unsharding #94 (comment)). (This may already happen as part of Tests for unsharding PR #99.)
• Reevaluate after reading the PR changing the threshold timeout calculations (Size-based unsharding #94 (comment)).

[schomatis]

Done.