Add an RPC endpoint to fetch the scheduled transactions

[wanderingbort]

This adds an RPC endpoint /v1/chain/get_scheduled_transactions which will produce a list of the scheduled but not yet executed/failed/expired transactions according to the chain data.

Inputs can include:

{ 
   "lower_bound" : ISO timestamp or transaction Id for the first entry desired
   "limit" : the maximum number of transactions to return
   "json" : true/false whether the packed transaction is converted to json 
}

the result is an object with:

{
   "transactions" : [ .. an array of the qualifying scheduled transactions .. ] 
   "more" : the transaction id to use as a lower bound for the next "page" of results
}

For example:

$ curl -d '{"limit":5,"json":false}' 'http://localhost/v1/chain/get_scheduled_transactions' | json_pp

{
   "transactions" : [
      {
         "delay_until" : "2018-07-03T17:49:54.500",
         "published" : "2018-06-30T17:49:54.500",
         "sender_id" : "0xa09864ff499bbd660000000000000000",
         "trx_id" : "80e87f10f8cb5588d465a97bb1605f06dd77ab3683b94363224c819f39a17c6d",
         "payer" : "guytqmjzgmge",
         "sender" : "eosio",
         "expiration" : "2018-07-03T17:59:54.500",
         "transaction" : "c3c2375b15e5948e1ca7000080e90f00010000000000ea305500000000a4a997ba01a09864ff499bbd6600000000a8ed323208a09864ff499bbd6600"
      },
      {
         "transaction" : "22c3375bd4e558de27ca000080e90f00010000000000ea305500000000a4a997ba01a09861f74c98856700000000a8ed323208a09861f74c98856700",
         "expiration" : "2018-07-03T18:01:30.000",
         "sender" : "eosio",
         "payer" : "gy2tknbrgage",
         "published" : "2018-06-30T17:51:30.000",
         "delay_until" : "2018-07-03T17:51:30.000",
         "sender_id" : "0xa09861f74c9885670000000000000000",
         "trx_id" : "53c18bdcb0b2e4053ddd992af6cb4bef1dfd7a42db2c714c090f95b8897022c5"
      },
      {
         "sender" : "eosio",
         "expiration" : "2018-07-03T18:05:00.500",
         "transaction" : "f5c3375b79e7361c33da000080e90f00010000000000ea305500000000a4a997ba01a09861fe4f94076100000000a8ed323208a09861fe4f94076100",
         "published" : "2018-06-30T17:55:00.500",
         "delay_until" : "2018-07-03T17:55:00.500",
         "trx_id" : "17a4d33a3e19c9ae9525ce77ce887f0bb09508c67a5acad87cd5c83f3b892875",
         "sender_id" : "0xa09861fe4f9407610000000000000000",
         "payer" : "g43tcnzygage"
      },
      {
         "sender_id" : "0xa09863fe4f9a88620000000000000000",
         "published" : "2018-06-30T17:55:03.500",
         "delay_until" : "2018-07-03T17:55:03.500",
         "trx_id" : "d89b04b0ce2af3fae12b064602f978f06086061c2ba7af1336b1a6071e25c98b",
         "payer" : "ge4donzygige",
         "sender" : "eosio",
         "expiration" : "2018-07-03T18:05:03.500",
         "transaction" : "f8c3375b7fe7cf21a8ff000080e90f00010000000000ea305500000000a4a997ba01a09863fe4f9a886200000000a8ed323208a09863fe4f9a886200"
      },
      {
         "published" : "2018-06-30T17:58:05.500",
         "sender_id" : "0xa0a662f64e9bbe650000000000000000",
         "delay_until" : "2018-07-03T17:58:05.500",
         "trx_id" : "3c1910fd6e9f980d9216fb8c057cf22403c59ffc32731e61de436b12cd4c4c0a",
         "payer" : "gqzdqnrqgene",
         "sender" : "eosio",
         "expiration" : "2018-07-03T18:08:05.500",
         "transaction" : "aec4375bebe853e9f16c000080e90f00010000000000ea305500000000a4a997ba01a0a662f64e9bbe6500000000a8ed323208a0a662f64e9bbe6500"
      }
   ],
   "more" : "8f67a8e5856c7377c35172876ea0070aeb3538018f73351413db8d1dad83a981"
}

[heifner]

Discussed a few improvements via DM.