Add go generate directive

At this stage this is just a comment for users. The directive will have
effect at go tip when go CL125580044 is submitted. When that happens, at
tip parser code generation can be acheived in the default case with:

    go generate

in the relevant package directory.
Merge pull request #142 from kortschak/generate

Add go generate directive
Add generate help for memstore/b generation
Generate cznic/b code as described in Makefile
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 #143 from kortschak/generate

Add generation support for B+tree code
Merge pull request #145 from kortschak/gremlin

Fix gremlin timeout handling
Merge branch 'master' into cassandra_backend