race condition in REPL crash fix

race.txt

WARNING: DATA RACE
Write by goroutine 12:
  github.com/google/cayley/query/gremlin.func·011()
      /home/daniel/Development/src/github.com/google/cayley/query/gremlin/session.go:143 +0x1d1

Previous read by ****
id : /en/aries_spears
goroutine 11:
  github.com/google/cayley/query/gremlin.runIteratorOnSession()
      /home/daniel/Development/src/github.com/google/cayley/query/gremlin/finals.go:256 +0x1fa
  github.com/google/cayley/query/gremlin.func·004()
      /home/daniel/Development/src/github.com/google/cayley/query/gremlin/finals.go:44 +0x2d3
  github.com/robertkrimen/otto.(*_object).call()
      /home/daniel/Development/src/github.com/robertkrimen/otto/type_function.go:140 +0x561
  github.com/robertkrimen/otto.(*_runtime).cmpl_evaluate_nodeCallExpression()
      /home/daniel/Development/src/github.com/robertkrimen/otto/cmpl_evaluate_expression.go:240 +0x92c
****
id : _:21354
  github.com/robertkrimen/otto.(*_runtime).cmpl_evaluate_nodeExpression()
      /home/daniel/Development/src/github.com/robertkrimen/otto/cmpl_evaluate_expression.go:44 +0xa76
  github.com/robertkrimen/otto.(*_runtime).cmpl_evaluate_nodeStatement()
      /home/daniel/Development/src/github.com/robertkrimen/otto/cmpl_evaluate_statement.go:59 +0x14c1
  github.com/robertkrimen/otto.(*_runtime).cmpl_evaluate_nodeStatementList()
      /home/daniel/Development/src/github.com/robertkrimen/otto/cmpl_evaluate_statement.go:118 +0xf7
  github.com/robertkrimen/otto.(*_runtime).cmpl_evaluate_nodeProgram()
      /home/daniel/Development/src/github.com/robertkrimen/otto/cmpl_evaluate.go:17 +0x1d5
  github.com/robertkrimen/otto.func·040()
      /home/daniel/Development/src/github.com/robertkrimen/otto/runtime.go:293 +0x7b
  github.com/robertkrimen/otto.catchPanic()
      /home/daniel/Development/src/github.com/robertkrimen/otto/error.go:243 +0xb1
  github.com/robertkrimen/otto.(*_runtime).cmpl_run()
      /home/daniel/Development/src/github.com/robertkrimen/otto/runtime.go:294 +0x1a4
  github.com/robertkrimen/otto.Otto.Run()
      /home/daniel/Development/src/github.com/robertkrimen/otto/otto.go:292 +0x89
  github.com/google/cayley/query/gremlin.(*Session).runUnsafe()
      /home/daniel/Development/src/github.com/google/cayley/query/gremlin/session.go:162 +0x425
=> <nil>
  github.com/google/cayley/query/gremlin.(*Session).ExecInput()
      /home/daniel/Development/src/github.com/google/cayley/query/gremlin/session.go:176 +0x3a4

Goroutine 12 (running) created at:
  github.com/google/cayley/query/gremlin.(*Session).runUnsafe()
      /home/daniel/Development/src/github.com/google/cayley/query/gremlin/session.go:152 +0x2a2
  github.com/google/cayley/query/gremlin.(*Session).ExecInput()
      /home/daniel/Development/src/github.com/google/cayley/query/gremlin/session.go:176 +0x3a4

Goroutine 11 (running) created at:
  github.com/google/cayley/db.Run()
      /home/daniel/Development/src/github.com/google/cayley/db/repl.go:56 +0x1eb
  github.com/google/cayley/db.Repl()
      /home/daniel/Development/src/github.com/google/cayley/db/repl.go:155 +0xca3
  main.main()
      /home/daniel/Development/src/github.com/google/cayley/cayley.go:164 +0x13b8
==================