Implement eth_getTransactionByBlockNumberAndIndex

[austinabell]

• Implements Implement eth_getTransactionByBlockNumberAndIndex #44 transaction by block number and index

Similar logic to the full tx decoding done in #87

To test:

1. Run the Ethermint node:

make install 
rm -rf ~/.emint*
emintd init moniker --chain-id 3
emintcli config chain-id 3
emintcli config output json
emintcli config indent true
emintcli config trust-node true
emintcli keys add austin
testpass
testpass
emintcli emintkeys add austineth
testpass
testpass
emintd add-genesis-account $(emintcli keys show austin -a) 1000photon,100000000stake
emintd add-genesis-account $(emintcli emintkeys show austineth -a) 100000000photon,100000000stake
emintd gentx --name austin
testpass
emintd collect-gentxs
emintd validate-genesis
emintd start --pruning=nothing --rpc.unsafe --log_level "main:info,state:info,mempool:info,*:error"

2. Start rest server with unlocked key to send tx to test functionality

emintcli rest-server --laddr "tcp://localhost:8545" --unlock-key austineth

3. Send a transaction to test all values in a block:

curl -X POST --data '{"jsonrpc":"2.0","method":"eth_sendTransaction","params":[{"from":"0x'$(echo -n $(emintcli emintkeys show austineth -w))'","to":"0xd46e8dd67c5d32be8058bb8eb970870f07244567","gas":"0x76c0","gasPrice":"0x12","value":"0x20","data":"0xd46e8dd67c5d32be8d46e8dd67c5d32be8058bb8eb970870f072445675058bb8eb970870f072445675"}],"id":1}' -H "Content-Type: application/json" http://localhost:8545/rpc

Retrieve transaction by block number and index (replace 0x2 and 0x0):

curl -X POST --data '{"jsonrpc":"2.0","method":"eth_getTransactionByBlockNumberAndIndex","params":["0x2", "0x0"],"id":1}' -H "Content-Type: application/json" http://localhost:8545/rpc
{"jsonrpc":"2.0","id":1,"result":{"blockHash":"0x048091bc7ddc283f77bfbf91d73c44da58c3df8a9cbc867405d8b7f3daada22f","blockNumber":"0x2","from":"0xd06b1d4c999dd69f9e6334baf047e398528bd4a7","gas":"0x76c0","gasPrice":"0x12","hash":"0x1b25646fcda0f30aa352e3d2825d0f5eef6e87d03f34253596b1197b1a7be050","input":"0xd46e8dd67c5d32be8d46e8dd67c5d32be8058bb8eb970870f072445675058bb8eb970870f072445675","nonce":"0x0","to":"0xd46e8dd67c5d32be8058bb8eb970870f07244567","transactionIndex":"0x0","value":"0x20","v":"0x2a","r":"0xc34947a181c44a0bf26817682025280f6673ced03af59604ece3a671435b2209","s":"0x3bb128d85d883136b2195fd224581e363105870eb14d1d894487c7a69691357e"}}

and you can check invalid indexes to see it returns null