channeldb: don't use iota for the ChannelType enum
channeldb: don't use iota for the ChannelType enum

This commit modifies the ChannelType enum to no longer use iota as
changes in the definition would cause the values to shift, breaking the
long-term stability required for persistence. Instead, we now select
values manually to indicate the particular channel type.

bryanvu/lnd,ddustin/lnd,ddustin/lnd,aakselrod/lnd,bryanvu/lnd,LightningNetwork/lnd,AndrewSamokhvalov/lnd,aakselrod/lnd,LightningNetwork/lnd,AndrewSamokhvalov/lnd