Use chain registry for denom data

[mircea-c]

GetDenomMetadata function uses the osmosis asset list to fetch denom metadata. This list is incomplete and seems to not have a stable naming convention.

To fix this we will:

• replace the url with one pointing to the chain-registry asset list which contains more detailed denom information and is actively and predictably maintained
• fetch denom metadata only once for each chain
• throw an exception if the denom cannot be found in the list, instead of default 6 digit precision

[mircea-c]

Closed by #65