Expand DEX fee capability

[prasannavl]

Enable pool dex in and dex out are set to the following with the highest priority of these:

=== P1 (Already exists) 
/v0/poolpairs/<x>/token_a_fee_pct
/v0/poolpairs/<x>/token_b_fee_pct

== P2 (New - to be implemented)
/v0/token/<x>/dex_in_fee_pct => Apply to all pools, unless overriden by above.
/v0/token/<x>/dex_out_fee_pct => Apply to all pools, unless overriden by above.

=== Don't see real use case, can skip it for now. 
/v0/poolpairs/global/in_fee_pct => Apply to all pools, unless overriden by above
/v0/poolpairs/global/out_fee_pct => Apply to all pools, unless overriden by above

This enables DEX fees to be set per pool, or per token (to apply to all pools with the token), with per pool taking priority if set.

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