Switch to btcutil.Amount.

While here, remove the use of btcutil amount constants, as the string
conversions are now handled by btcutil.Amount's String method.

ok @davecgh
IMPORT PATH MODIFICATION CHANGESET
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Please use this changeset as the first one on top of the conformal codebase.
Note that if there are new imports, they need to be changed as part of this
changeset. In general, use "git rebase" on github.com/conformal's version
and apply this changeset first.