A proposal to pave the way for denser housing types across what have long been traditional single-family suburban neighborhoods in Littleton has hit a hard roadblock, with the City Council voting to postpone consideration of the measure indefinitely. Tuesday night’s vote to set aside the proposed amendment to the city’s land use code was 6-1, with Mayor Pro Tem Stephen Barr casting the lone no vote. Councilwoman Pam Grove said that in her five years on the council, “never has an issue hit such a hot button.” The ordinance aimed to make it easier to bring to Littleton “missing middle” types of housing — structures like duplexes, triplexes and accessory dwelling units — in all residential zoning districts in the city.