Feature request: Add sector ID to logs

[karalabe]

Currently all the sealing logs are context-less. If I'm running 7-10 different sectors concurrently on the same machine, it's impossible to see which log belongs to which sector. When something fails, it's almost impossible to gather back the trace. It's also super hard to see which sector is at what progress. E.g.

/src/github.com-1ecc6299db9ec823/storage-proofs-porep-5.1.4/src/stacked/vanilla/proof.rs:363","msg":"  generated layer 8 store with id layer-8"}
/src/github.com-1ecc6299db9ec823/storage-proofs-porep-5.1.4/src/stacked/vanilla/proof.rs:368","msg":"  setting exp parents"}
/src/github.com-1ecc6299db9ec823/storage-proofs-porep-5.1.4/src/stacked/vanilla/proof.rs:320","msg":"generating layer: 9"}
/src/github.com-1ecc6299db9ec823/storage-proofs-porep-5.1.4/src/stacked/vanilla/proof.rs:363","msg":"  generated layer 8 store with id layer-8"}
/src/github.com-1ecc6299db9ec823/storage-proofs-porep-5.1.4/src/stacked/vanilla/proof.rs:368","msg":"  setting exp parents"}
/src/github.com-1ecc6299db9ec823/storage-proofs-porep-5.1.4/src/stacked/vanilla/proof.rs:320","msg":"generating layer: 9"}
/src/github.com-1ecc6299db9ec823/storage-proofs-porep-5.1.4/src/stacked/vanilla/proof.rs:363","msg":"  generated layer 4 store with id layer-4"}
/src/github.com-1ecc6299db9ec823/storage-proofs-porep-5.1.4/src/stacked/vanilla/proof.rs:368","msg":"  setting exp parents"}
/src/github.com-1ecc6299db9ec823/storage-proofs-porep-5.1.4/src/stacked/vanilla/proof.rs:320","msg":"generating layer: 5"}
/src/github.com-1ecc6299db9ec823/storage-proofs-porep-5.1.4/src/stacked/vanilla/proof.rs:363","msg":"  generated layer 4 store with id layer-4"}
/src/github.com-1ecc6299db9ec823/storage-proofs-porep-5.1.4/src/stacked/vanilla/proof.rs:368","msg":"  setting exp parents"}
/src/github.com-1ecc6299db9ec823/storage-proofs-porep-5.1.4/src/stacked/vanilla/proof.rs:320","msg":"generating layer: 5"}
/src/github.com-1ecc6299db9ec823/storage-proofs-porep-5.1.4/src/stacked/vanilla/proof.rs:354","msg":"  storing labels on disk"}
/src/github.com-1ecc6299db9ec823/storage-proofs-porep-5.1.4/src/stacked/vanilla/proof.rs:363","msg":"  generated layer 8 store with id layer-8"}
/src/github.com-1ecc6299db9ec823/storage-proofs-porep-5.1.4/src/stacked/vanilla/proof.rs:368","msg":"  setting exp parents"}
/src/github.com-1ecc6299db9ec823/storage-proofs-porep-5.1.4/src/stacked/vanilla/proof.rs:320","msg":"generating layer: 9"}
/src/github.com-1ecc6299db9ec823/storage-proofs-porep-5.1.4/src/stacked/vanilla/proof.rs:354","msg":"  storing labels on disk"}
/src/github.com-1ecc6299db9ec823/storage-proofs-porep-5.1.4/src/stacked/vanilla/proof.rs:354","msg":"  storing labels on disk"}
/src/github.com-1ecc6299db9ec823/storage-proofs-porep-5.1.4/src/stacked/vanilla/proof.rs:363","msg":"  generated layer 8 store with id layer-8"}

Would be nice to pass the sector ID through to the logger so each log could be tagged with the sector it belongs to.

[dignifiedquire]

@vmx wanna take a look at adding some context to our logs?