Treat bare slash as query string term

[cyphercodes]

Summary

• Treat a single escaped / query-string term as a match term instead of an empty slash-delimited regexp.
• Add parser and search coverage for querying a path field with /.

Tests

• gofmt -w search/query/query_string.y.go search/query/query_string_parser_test.go search_test.go
• go test ./search/query -run 'TestQuerySyntaxParserValid' -count=1
• go test . -run 'TestQueryStringSearchEscapedSlashPath|TestQueryStringEmptyConjunctionSearcher' -count=1
• go test ./search/query . -count=1
• go vet ./search/query .
• git diff --check

Fixes #2362

Note: CONTRIBUTING.md says Couchbase CLA applies.

[coveralls]

coverage: 52.195% (-0.01%) from 52.206% — cyphercodes:fix-2362-bare-slash-query into blevesearch:master