Fix getchaintxstats()

1. Calculate nblocks more adaptive.
   If not specify nblocks-parameter, illegal parameter error
   will happen when target block height is below blocks for 1 month.
   To avoid this error, set default nblocks to
   min(blocks for 1 month, target block's height - 1)
   And allowing 0 so that this RPC works good even if target block is
   genesis block or 1st block.
2. Correct error message.
   nblocks accepts [0 .. block's height -1] . so fix as following:
   "Invalid block count: should be between 0 and the block's height - 1"
3. Add check 0-divide.
   If nTimeDiff = 0 then returns {... "txrate":} and
   bitcoin-cli cannot handle the response.
   To avoid this error, do not return "txrate" if nTimeDiff = 0.
4. Add following 3 elements to the return object.
   1) 'window_block_count' : Size of the window in number of blocks.
   2) 'window_tx_count' : The number of transactions in the window.
   3) 'window_interval' : The elapsed time in the window.
   They clarify how 'txrate' is calculated. 2) and 3) are returned
   only if 'window_block_count' is a positive value.
5. Improve help text for 'time' as following.
   'The timestamp for the final block in the window in UNIX format.

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