Try to speed up dependency_hash()

[wlandau]

Ref: #647. Ideas:

• Avoid calls to config$cache$exists().
• Find an alternative to self_hash(). How do we reuse hashes without having to read them again from the cache?

[wlandau]

I think we can use in-memory memoization for self_hash(), as long as the memoized hashes do not carry over to the next make(). self_hash() seems to only be called inside dependency_hash(), so it is only used to get up-to-date hashes. If we can borrow some of those hashes across different targets, we will be in great shape.

[wlandau]

Avoiding calls to config$cache$exists() turned out to not do much. Memoization can help, but we need to be careful. We should only memoize hashes if the targets are already checked and up to date, and we want to forget everything we memoized in the next session of make(). Even after that, there is danger. I tried this, and some of the tests still failed. Will attempt again at some point, hopefully soon.

[wlandau]

Useful: https://gist.github.com/hadley/6353939

[wlandau]

Current approach:

• Faster $driver$get_hash() for the RDS driver richfitz/storr#96
• 33% runtime reduction for $get_hash() in RDS storrs richfitz/storr#98
• Memoize config$cache$get_hash() on up-to-date targets #660

#660 is sufficient to close this issue for now. The rest can be handled through storr, I think.

[wlandau]

Fixed via #660. Other bottlenecks are now more pressing.