fix for high datarate devices

[tobiasdam]

in order to prevent devices from spamming and others from starving we need to:

• create a list consisting of separate queues for every device
• use sortedsets as those queues (to enforce unique messages)
• set expire key for messages

[tobiasdam]

expire cannot be used on fields of keys (>_ <)

[tobiasdam]

possible workaround:

• the zset only contains keys of redis string entries
• each message is a string entry and has the expire property
• the key of the message is stored in the zset along with the timestamp as score
• ~lorawan_tx~ (likely better) bridge-forwarder uses ZREMRANGEBYSCORE <zset_key> 0 <timestamp some time ago> to remove old entries in the zset

[tobiasdam]

this requires changes in lorawan_tx and bridge_forwarder

[tobiasdam]

implemented via 36d6b9969c57b78bd8d0ad822db1f4d2c224177b and 0973cf2da435ba4ec36b62ddbf562a689e92ba4c

[tobiasdam]

further enhancement (see #7):

• only deduplicate subsequent messages
• implement a non-buffered mode
• use non-buffered mode as default for router devices
• implement a setting for the mode in the webinterface