storage: more paranoid handling of bounds and palimpsests in mutable share files

storage: more paranoid handling of bounds and palimpsests in mutable share files
* storage server ignores requests to extend shares by sending a new_length
* storage server fills exposed holes (created by sending a write vector whose offset begins after the end of the current data) with 0 to avoid "palimpsest" exposure of previous contents
* storage server zeroes out lease info at the old location when moving it to a new location
ref. #1528