* [sonic_platform.sfp] support get_transceiver_dom_threshold_info_dict
* [platform/sfp]qsfp threshold and beautify code
1. qsfp threshold: tx power
2. beautify code, removing some magic numbers
3. optimize get_present by only reading one byte.
optimize SFP module operations and fix issues.
- split initialization of variant categories of devices and initialize each category of devices only when needed, so that unnecessary dependencies can be avoided.
- update watchdog logic, only initializing watchdog when referenced.
- support platform.py and enable to initialize variant devices on a host/docker basis
- update init so that sonic_platform can be imported as a whole.
* [Mellanox]refractor the sfp event change notification logic for new platform api
remove the standalong daemon which is in charge of polling sfp change event through sdk interface
and move the polling stuff to the event in the chassis daemon.
* rephase some comment
* fix typo in sfp_event.sfp_event.initialize
Variables SFP_VLOT_OFFSET and QSFP_VLOT_OFFSET containing the typo are originally defined in repo sonic-platform-common. The typo has been fixed in PR #33. However, some Mellanox-specific code hasn't updated correspondingly, which results in xcvrd fail to start.
This PR updates the variable name in Mellanox-specific code correspondingly to fix that.
