Fix staticcheck errors

[SuperQ]

I ran the staticcheck tool and discovered a few issues. It would be nice to enable this in the Makefile like we do with several of the exporters.

Here's the current list of issues:

cmd/prometheus/main.go:233:15: the channel used with signal.Notify should be buffered (SA1017)
discovery/kubernetes/node.go:191:46: api.NodeLegacyHostIP is deprecated: NodeLegacyHostIP will be removed in 1.7.  (SA1019)
documentation/examples/remote_storage/remote_storage_adapter/main.go:97:33: prometheus.Handler is deprecated: Please note the issues described in the doc comment of InstrumentHandler. You might want to consider using promhttp.Handler instead (which is not instrumented).  (SA1019)
retrieval/target_test.go:133:64: cutset contains duplicate characters (SA1024)
storage/local/codable/codable.go:69:13: argument should be one word large or less to avoid allocations (SA6002)
storage/local/mapper_test.go:65:10: this value of err is never used (SA4006)
storage/local/persistence.go:465:27: os.SEEK_CUR is deprecated: Use io.SeekStart, io.SeekCurrent, and io.SeekEnd.  (SA1019)
storage/local/persistence.go:494:16: argument should be one word large or less to avoid allocations (SA6002)
storage/local/persistence.go:501:68: os.SEEK_SET is deprecated: Use io.SeekStart, io.SeekCurrent, and io.SeekEnd.  (SA1019)
storage/local/persistence.go:564:71: os.SEEK_SET is deprecated: Use io.SeekStart, io.SeekCurrent, and io.SeekEnd.  (SA1019)
storage/local/persistence.go:817:51: os.SEEK_SET is deprecated: Use io.SeekStart, io.SeekCurrent, and io.SeekEnd.  (SA1019)
storage/local/persistence.go:974:52: os.SEEK_SET is deprecated: Use io.SeekStart, io.SeekCurrent, and io.SeekEnd.  (SA1019)
storage/local/persistence.go:1010:43: os.SEEK_SET is deprecated: Use io.SeekStart, io.SeekCurrent, and io.SeekEnd.  (SA1019)
storage/local/persistence.go:1036:35: os.SEEK_CUR is deprecated: Use io.SeekStart, io.SeekCurrent, and io.SeekEnd.  (SA1019)
storage/local/persistence.go:1663:16: argument should be one word large or less to avoid allocations (SA6002)
storage/local/persistence_test.go:138:21: this value of err is never used (SA4006)
storage/local/storage_test.go:1423:2: this value of series is never used (SA4006)
storage/remote/client_test.go:67:6: this value of err is never used (SA4006)
storage/remote/queue_manager.go:252:21: using time.Tick leaks the underlying ticker, consider using it only in endless functions, tests and the main package, and use time.NewTicker here (SA1015)
web/web.go:186:12: prometheus.InstrumentHandlerFunc is deprecated: InstrumentHandlerFunc is deprecated for the same reasons as InstrumentHandler is.  (SA1019)
web/web.go:204:35: prometheus.Handler is deprecated: Please note the issues described in the doc comment of InstrumentHandler. You might want to consider using promhttp.Handler instead (which is not instrumented).  (SA1019)

[lock]

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