invalid insertion backlog count when using flows (trac #2535)

[behrisch]

in an attempt to conserve resources, flow vehicles are only created upon successful insertion of the previous vehicle. However, all currently pending vehicles are created at once after a successful insertion, defeating the purpose of resource conservation.

Migrated from http://sumo.dlr.de/ticket/2535

{
    "status": "closed", 
    "changetime": "2016-08-29T12:22:20Z", 
    "description": "in an attempt to conserve resources, flow vehicles are only created upon successful insertion of the previous vehicle. However, all currently pending vehicles are created at once after a successful insertion, defeating the purpose of resource conservation.", 
    "reporter": "namdre", 
    "cc": "", 
    "resolution": "fixed", 
    "_ts": "1472473340322829", 
    "component": "other (also unclassified)", 
    "summary": "invalid insertion backlog count when using flows", 
    "priority": "major", 
    "keywords": "", 
    "time": "2016-08-29T11:36:12Z", 
    "milestone": "0.28.0", 
    "owner": "namdre", 
    "type": "defect"
}

[behrisch]

@namdre committed 5173ea3 (aka r21349):
added failing test refs #2535

[behrisch]

• @namdre committed 0137e5d (aka r21350):
  fix invalid insertion backlog count when using flows (trac #2535) #2535
• @namdre changed owner from "" to "namdre"
• @namdre changed resolution from "" to "fixed"
• @namdre changed status from "new" to "closed"

[behrisch]

@namdre committed afb1070 (aka r21351):
patching expected results refs #2535, refs #21