Pkg.jl: Excessive writes to `manifest_usage.toml`

[ericphanson]

I instantiated a manifest (on Julia 1.3.1) and it added 9 entries (for the same manifest at very close timestamps) to .julia/logs/manifest_usage.toml.

I had a bunch of corrupted entries in there (probably from interrupting Pkg operations) which caused Pkg.gc() to fail, and it seems like maybe there would be fewer opportunities for corrupted entries if Pkg didn't write to that file so much.

(I first reported this on Slack).

[ericphanson]

It might be nice also if interrupt exceptions during these writes were handled somehow so as to not corrupt the toml file.

[KristofferC]

Yeah, and also handling corrption in the file better (trying to recover)