scorch docNumberToBytes() checks cap(buf) before allocating
scorch docNumberToBytes() checks cap(buf) before allocating

With more pprof focusing (zooming in on a particular func), there were
still some memory allocations showing up with docNumberToBytes() in
micro benchmarks of bleve-query.  On a dev macbook, on an index of 50K
wikipedia docs, using search of relatively common "text:date"...

   400 qps - upsidedown/moss
   680 qps - scorch before
   775 qps - scorch after

blevesearch/bleve,steveyen/bleve,steveyen/bleve,blevesearch/bleve