Prepare gremlin for introduction of a worker type
Refactor work out into worker type
Mark limit as unused in gremlin and SEXP

Used in SEXP. Currently there appears to be no way to limit the number
of query returns from MQL.
Remove unnecessary indirection
Fix worker termination handling

Fixes issue #102.
Fix build
Inconsequential whitespace changes
Merge pull request #145 from kortschak/gremlin

Fix gremlin timeout handling