Default cache paths to use FIL_PROOFS_CACHE_DIR if set.

[porcuquine]

Closes #1206.

NOTE: Do not merge until a decision is made. This PR was a proposal for @lanzafame's approval, but merging it may cause as much or more trouble as the originally reported problem.

UPDATE: We have settled on solution: separate actual cache usage from canonical parameter location.

In particular, we must ensure the location of the Groth parameter cache does not change for any user. (cc: @travisperson)

This is untested, but I wanted to push something quickly for @lanzafame to review, since this is high priority for him.

@lanzafame Please approve if this seems to satisfy your requirements. If so, we'll we might get this into the upcoming release after making sure it's solid.

TODO:

• document this change in README.
• document usage/policy for future cache settings in source.

[lanzafame]

It appears to perform what I would like it to

[travisperson]

Just to clarify (and in preparation for change log) I believe this is how it works now:

If FIL_PROOFS_PARENT_CACHE is set cached parents trees will be stored under $FIL_PROOFS_PARENT_CACHE. Setting FIL_PROOFS_PARENT_CACHE FIL_PROOFS_CACHE_DIR has no effect.
If FIL_PROOFS_PARENT_CACHE is unset and FIL_PROOFS_CACHE_DIR is set, cached parents trees will be stored under $FIL_PROOFS_CACHE_DIR/filecoin-parents.
If If FIL_PROOFS_CACHE_DIR is unset and FIL_PROOFS_PARENT_CACHE is unset, the directory will be /var/tmp/filecoin-parents

The behaviour of the parameter cache is unchanged. Setting FIL_PROOFS_CACHE_DIR has no affect on their location.

[porcuquine]

@travisperson I think this looks right except for one thing:

If FIL_PROOFS_PARENT_CACHE is set cached parents trees will be stored under $FIL_PROOFS_PARENT_CACHE. Setting FIL_PROOFS_PARENT_CACHE has no affect.

Did you mean setting FIL_PROOFS_CACHE_DIR has no effect?

[travisperson]

Yes

[cryptonemo]

Approved, although no longer related to TMPDIR. Will that part cause confusion?

[porcuquine]

@cryptonemo Fixed reference to TMPDIR. If you have bandwidth to propagate changes everywhere after #1211 merges (README, config sample, etc.) that would be helpful. If so, I consider this can merge and be incorporated in the next release when ready. (I will update the description to reflect this decision.)

[porcuquine]

@cryptonemo I pushed one more commit documenting usage in a comment. Please review before propagating, since it clarifies a wart: specifically, the new env var cannot be set by config file — just env var.