Refactor dht_node.rs

[NamsooCho]

Refactor dht_node.rs.

1. Revise codes according to changes of packet.rs
2. Introduce Precomputed HashMap
3. Move Option to Result if available

[coveralls]

Pull Request Test Coverage Report for Build 36

• 756 of 833 (90.76%) changed or added relevant lines in 6 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage decreased (-0.5%) to 96.59%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/toxcore/dht_new/packet.rs	105	111	94.59%
src/toxcore/dht_new/codec.rs	34	49	69.39%
src/toxcore/dht_new/dht_impl.rs	120	144	83.33%
src/toxcore/dht_new/dht_node.rs	455	487	93.43%

Files with Coverage Reduction	New Missed Lines	%
src/toxcore/dht_new/packet.rs	1	98.15%

Totals
Change from base Build 11:	-0.5%
Covered Lines:	6514
Relevant Lines:	6744

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💛 - Coveralls