vpcs: Protect against race conditions in IP range assignment.

[andrewsomething]

This prevents parallel creation of VPCs in the same region to protect against race conditions in IP range assignment. Fixes: #551

It creates an internal copy of the mutexkv package that had previously been part of the Terraform SDK. This is the same strategy employed by other providers.

https://www.terraform.io/docs/extend/guides/v2-upgrade-guide.html#removal-of-helper-mutexkv-package

Providers that need the functionality provided by the helper/mutexkv package are encouraged to copy the types and functions it provided into their own codebase, provided here under a public domain license

Before change, the new test fails with:

$ make testacc TESTARGS='-run=TestAccDigitalOceanVPC_IPRangeRace'
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test $(go list ./... |grep -v 'vendor') -v -run=TestAccDigitalOceanVPC_IPRangeRace -timeout 120m
?       github.com/digitalocean/terraform-provider-digitalocean [no test files]
=== RUN   TestAccDigitalOceanVPC_IPRangeRace
=== PAUSE TestAccDigitalOceanVPC_IPRangeRace
=== CONT  TestAccDigitalOceanVPC_IPRangeRace
    resource_digitalocean_vpc_test.go:87: Step 1/1 error: Error running apply: 
        Error: Error creating VPC: POST https://api.digitalocean.com/v2/vpcs: 422 (request "beeb46fc-d4ba-4223-bffd-462e21fe4f23") This range/size overlaps with another VPC network in your account: tf-acc-test-rk10vpxzml 10.108.0.0/20.
        
        
--- FAIL: TestAccDigitalOceanVPC_IPRangeRace (2.72s)
FAIL
FAIL    github.com/digitalocean/terraform-provider-digitalocean/digitalocean    2.730s
testing: warning: no tests to run
PASS
ok      github.com/digitalocean/terraform-provider-digitalocean/internal/datalist       (cached) [no tests to run]
testing: warning: no tests to run
PASS
ok      github.com/digitalocean/terraform-provider-digitalocean/internal/setutil        (cached) [no tests to run]
FAIL
make: *** [GNUmakefile:16: testacc] Error 1

Post-change it passes:

$ make testacc TESTARGS='-run=TestAccDigitalOceanVPC_IPRangeRace'
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test $(go list ./... |grep -v 'vendor') -v -run=TestAccDigitalOceanVPC_IPRangeRace -timeout 120m
?       github.com/digitalocean/terraform-provider-digitalocean [no test files]
=== RUN   TestAccDigitalOceanVPC_IPRangeRace
=== PAUSE TestAccDigitalOceanVPC_IPRangeRace
=== CONT  TestAccDigitalOceanVPC_IPRangeRace
--- PASS: TestAccDigitalOceanVPC_IPRangeRace (8.24s)
PASS
ok      github.com/digitalocean/terraform-provider-digitalocean/digitalocean    8.245s
testing: warning: no tests to run
PASS
ok      github.com/digitalocean/terraform-provider-digitalocean/internal/datalist       (cached) [no tests to run]
testing: warning: no tests to run
PASS
ok      github.com/digitalocean/terraform-provider-digitalocean/internal/mutexkv        (cached) [no tests to run]
testing: warning: no tests to run
PASS
ok      github.com/digitalocean/terraform-provider-digitalocean/internal/setutil        (cached) [no tests to run]

[ChiefMateStarbuck]

NOICE