Make timer work for wasm module

[bianpengyuan]

This includes several changes to make timer work for wasm module:

• instead of copying dispatcher from base wasm, copy the dispatcher from filter construction function. dispatcher from base wasm cannot be used in thread local context due to thread safe check in envoy.
• remove tick_ function pointer, seems to be usefulness.
• add ontick and set period into wasm intrinsic api.

[istio-testing]

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[bianpengyuan]

@jplevyak Could you please take a look? This is minimal change to make ticker work for wasm module.

[bianpengyuan]

@jplevyak Could you help me merge this or grant me write access to this repo? Thanks!