Fail to broadcast a transaction: minimum relay fee not met

[nondiremanuel]

During the last phase of tests, there was an occurrence of not being able to relay a transaction because the minimum fee was not met.

Since it is not clear where the rounding/estimation issue comes from (not clear if #1132 or else), we agreed that if after a quick further investigation the origin is still unknown, we will go with a temporary patch for v7, i.e. either using 1.1 sats/vb as a multiplier when 1 sats/vb is selected or adding 10 sats to the total fee amount if 1 sats/vb was selected. This should give some margin (at a small cost) to avoid that the user is not able to relay the transaction. In such case, a more detailed fix would then be released in a followup.