[mqtt] Import mqtt-broker project (Azure#2623)

Import mqtt-broker project from its own repo
- location: ./mqtt
- disable dotnet and edgelet checks for mqtt-broker
- import mqtt3 client project
- import mqtt3-fuzz project
- make it compile on windows

.gitignore

@@ -322,3 +322,7 @@ stylecop.json
 *.swp
 *.swo
 *.vi
+
+# fuzz
+in
+out

builds/checkin/dotnet.yaml

@@ -9,14 +9,14 @@ jobs:
 ################################################################################
   - job: check_run_pipeline
 ################################################################################
-    displayName: Check pipeline preconditions (changes ARE NOT in edgelet and docs)
+    displayName: Check pipeline preconditions (changes ARE NOT in either edgelet, docs, or mqtt folder)
     pool:
       vmImage: "ubuntu-16.04"
     steps:
       - bash: |
-          git log -m -1 --name-only --first-parent --pretty="" | egrep -i -v '^(edgelet|doc)'
+          git log -m -1 --name-only --first-parent --pretty="" | egrep -i -v '^(edgelet|doc|mqtt)'
           if [[ $? == 0 ]]; then
-            echo "Detected changes outside of edgelet and docs folders"
+            echo "Detected changes outside of edgelet, docs or mqtt folders"
             echo "##vso[task.setvariable variable=RUN_PIPELINE;isOutput=true]TRUE"
           fi
         displayName: Check changes in sources
@@ -124,4 +124,4 @@ jobs:
           checkCoverage: true
           coverageFailOption: fixed
           coverageType: lines
-          coverageThreshold: $(coverage.goal)
+          coverageThreshold: $(coverage.goal)