Update chainTxData

[bitspill]

Improves initial sync progress estimation, otherwise early sync progress is reported as extremely slow increase per hour with the final minutes seemingly increasing at a rate of ~500% per hour -- estimates were calculating 3 transactions per second rather than the actual 1 transaction per 10 seconds.

Mainnet as of block 475,000

getchaintxstats 43200 000000000000a72545994ce72b25042ea63707fca169ca4deb7f9dab4f1b1798
{
  "time": 1543817453,
  "txcount": 2033711,
  "window_block_count": 43200,
  "window_tx_count": 274083,
  "window_interval": 2611099,
  "txrate": 0.1049684443217205
}

Testnet as of block 90,000

getchaintxstats 43200 00000023b66f46d74890287a7b1157dd780c7c5fdda2b561eb96684d2b39d62e
{
  "time": 1543633332,
  "txcount": 146666,
  "window_block_count": 43200,
  "window_tx_count": 56233,
  "window_interval": 2826119,
  "txrate": 0.01989760516099994
}

[spyder46n2]

Thanks for looking at this, been on the list of things to look into for a while. Looks good.
Pulled into local repo and tested on Windows, Linux, and OSX.