refactor file cache code

[rfyiamcool]

what

1. add rwlock for file cache, to ensure thread-safe.
2. refactor file_test.go by assert mode.
3. mkdirall dir when new file cachego.

[faabiosr]

Thanks you so much for you contribution, however I added some points regarding the changes. Also because is such a small change, I recommend you to squash all commits, and follow the commit message instructions. https://github.com/faabiosr/cachego/blob/master/CONTRIBUTING.md#create-a-commit

[rfyiamcool]

Thanks you so much for you contribution, however I added some points regarding the changes. Also because is such a small change, I recommend you to squash all commits, and follow the commit message instructions. https://github.com/faabiosr/cachego/blob/master/CONTRIBUTING.md#create-a-commit

oh, done, i merge multiple commit to one commit.

[faabiosr]

Regarding the last commented about the removed tests. Also I recommend you to rebase this PR with the latest master changes (made by you 😄 ).

[faabiosr]

@rfyiamcool Did you rebase your branch with the latest changes?

[faabiosr]

By the way, thanks for the great contribution @rfyiamcool !