Restructure project

We previously didn't really had any structure in our project apart
from creating a new folder for each package in our project root.
Now that we have accumulated some packages, we use the well-known
Golang project layout in order to clearly communicate our intent
with packages. See https://github.com/golang-standards/project-layout

internal/runner/execution_storage.go

@@ -0,0 +1,46 @@
+package runner
+
+import (
+	"gitlab.hpi.de/codeocean/codemoon/poseidon/pkg/dto"
+	"sync"
+)
+
+// ExecutionStorage stores executions.
+type ExecutionStorage interface {
+	// Add adds a runner to the storage.
+	// It overwrites the existing execution if an execution with the same id already exists.
+	Add(id ExecutionID, executionRequest *dto.ExecutionRequest)
+
+	// Pop deletes the execution with the given id from the storage and returns it.
+	// If no such execution exists, ok is false and true otherwise.
+	Pop(id ExecutionID) (request *dto.ExecutionRequest, ok bool)
+}
+
+// localExecutionStorage stores execution objects in the local application memory.
+// ToDo: Create implementation that use some persistent storage like a database.
+type localExecutionStorage struct {
+	sync.RWMutex
+	executions map[ExecutionID]*dto.ExecutionRequest
+}
+
+// NewLocalExecutionStorage responds with an ExecutionStorage implementation.
+// This implementation stores the data thread-safe in the local application memory.
+func NewLocalExecutionStorage() *localExecutionStorage {
+	return &localExecutionStorage{
+		executions: make(map[ExecutionID]*dto.ExecutionRequest),
+	}
+}
+
+func (s *localExecutionStorage) Add(id ExecutionID, executionRequest *dto.ExecutionRequest) {
+	s.Lock()
+	defer s.Unlock()
+	s.executions[id] = executionRequest
+}
+
+func (s *localExecutionStorage) Pop(id ExecutionID) (*dto.ExecutionRequest, bool) {
+	s.Lock()
+	defer s.Unlock()
+	request, ok := s.executions[id]
+	delete(s.executions, id)
+	return request, ok
+}