node: Allow comma-separated arrays for secondary hosts and weights

This was missed in moving to StructOpt in commit 2f1fc7e

Note that other arguments that take multiple values do not accept
comma-separated values

store: Clarify a variable name in metadata::create_subgraph

graphql: Fix error in logic what timing info to log

all: Address review comments for load management

Comments are in Zac's review of
graphprotocol#1762