Server stopped working when Error in SetExecutionDone

[prakashdivyy]

Describe the bug
The leader of the raft will crash when there is error in SetExecutionDone when the job was deleted

To Reproduce
Steps to reproduce the behavior:

1. Create jobs that concurrent handling 3 jobs at the same time
2. One of the created job must delete one job from other job created. (example job_a will delete job_b and job_c)

Expected behavior
There is no error after set execution done

Screenshots

time="2020-09-07T13:48:38Z" level=info msg="agent: Calling AgentRun" job_name=some_job_name node="10.0.6.51:6868"
time="2020-09-07T13:48:38Z" level=warning msg="grpc: Received execution done for a deleted job" node=dkron-server-5d9d4d9555-b6vzk
time="2020-09-07T13:48:38Z" level=error msg="store: Error in SetExecutionDone" error="grpc: Received execution done for a deleted job" node=dkron-server-5d9d4d9555-b6vzk
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x8 pc=0x2090b2c]
goroutine 76191 [running]:
github.com/distribworks/dkron/v3/dkron.(*GRPCServer).ExecutionDone(0xc001017470, 0x35c8d20, 0xc0019b9890, 0xc0019b98c0, 0x0, 0x0, 0x0)
	/home/runner/work/dkron/dkron/dkron/grpc.go:199 +0xcec
github.com/distribworks/dkron/v3/plugin/types._Dkron_ExecutionDone_Handler(0x29f1780, 0xc001017470, 0x35c8d20, 0xc0019b9890, 0xc001bb4780, 0x0, 0x35c8d20, 0xc0019b9890, 0xc0018d6500, 0x26e)
	/home/runner/work/dkron/dkron/plugin/types/dkron.pb.go:1566 +0x217
google.golang.org/grpc.(*Server).processUnaryRPC(0xc00094b1e0, 0x35ea7e0, 0xc002ba3500, 0xc000d86a00, 0xc001017590, 0x4b50818, 0x0, 0x0, 0x0)
	/home/runner/go/pkg/mod/google.golang.org/grpc@v1.29.1/server.go:1082 +0x50a
google.golang.org/grpc.(*Server).handleStream(0xc00094b1e0, 0x35ea7e0, 0xc002ba3500, 0xc000d86a00, 0x0)
	/home/runner/go/pkg/mod/google.golang.org/grpc@v1.29.1/server.go:1405 +0xccb
google.golang.org/grpc.(*Server).serveStreams.func1.1(0xc0004fcd50, 0xc00094b1e0, 0x35ea7e0, 0xc002ba3500, 0xc000d86a00)
	/home/runner/go/pkg/mod/google.golang.org/grpc@v1.29.1/server.go:746 +0xa1
created by google.golang.org/grpc.(*Server).serveStreams.func1
	/home/runner/go/pkg/mod/google.golang.org/grpc@v1.29.1/server.go:744 +0xa1

Specifications:

• Docker image v3.0.5 running on kubernetes with 3 server and 3 agent