add some missing extension docs

engine-extensions/src/main/java/io/nosqlbench/engine/extensions/globalvars/globalvars.md

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+globalvars extension
+===================
+
+Allows access to the global object map from SharedState.gl_ObjectMap, which allows
+for cross-binding and cross-thread data sharing.
+
+```
+var result = globalvars.get("result");
+```

engine-extensions/src/main/java/io/nosqlbench/engine/extensions/http/http.md

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+http extension
+==============
+
+Allow access to HTTP URLs from within scripts, supporting both basic
+get and post methods. In all cases, the returned type is the full
+response object, from which the body content can be accessed.
+
+## Examples
+
+Get content from a URL into a string variable:
+
+```
+var response= http.get("http://google.com/")
+```
+
+Post an empty body to a URL, useful for state-changing calls where
+all of the control data is in the URL:
+
+```
+var response= http.post("http://some.server/path/to/resource?query=foobarbaz")
+```
+
+Post content to a URL, specifying the URL, content value, and content type:
+```
+var response= http.post("http://some.server/path/to/resource", "this is the data", "text/plain");
+```

nb/src/test/resources/scripts/async/extension_files.js

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+// Just an example
+var content = files.read("somefile.txt");

nb/src/test/resources/scripts/async/extension_globalstate.js

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+// Just an example
+var result= globalvars.put("result","OK")
+var result = globalvars.get("result");
+print("result="+result);

nb/src/test/resources/scripts/async/extension_http.js

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+var response = http.get("http://example.google.com")

nb/src/test/resources/scripts/async/extension_s3.js

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+// This requires active credentials, so it is disabled by default.
+// This still serves as an example
+// s3.uploadDirToUrl("testdata","s3://nb-extension-test/testdata1");

nb/src/test/resources/scripts/async/extension_shutdown_hook.js

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+shutdown.addShutdownHook('testfunc', function f() {
+    print("shutdown hook running");
+});