only promote proposal if outside of gc window

[andymck]

This adds a check to the promotion of proposals to consider if the proposal is within the GC window. If it is then dont select it. This is necessary as not doing so could lead to potential sync problems if the chain var poc_validator_ephemeral_key_timeout is modified.

[evanmcc]

https://github.com/helium/blockchain-core/blob/9c5284871d48bc018fa7d4de6a056a64eac10d74/src/ledger/v1/blockchain_ledger_v1.erl#L2346

There will need to be safety logic here to the tune of:

• if a) gateway challenges OR b) safey var is set, use whatever value is in the code
• else use 101, which was the value at time of shipping

[evanmcc]

please see #1335