Fix some issues in block producer

[leventeliu]

Tx flooding

The AddTx broadcasting needs a TTL control, or simply check local existence first.

DATARACE

Calling WaitGroup.Add in RPC handling goroutine main cause DATARACE: calls with a positive delta that occur when the counter is zero must happen before a Wait.
Use new WaitGroup instance while creating new sub-goroutines in a worker goroutine instead.
This also changes the broadcasting mode from nonblocking to blocking the worker goroutine -- but will be fine with the timeout context, if they work properly.

Nil field

Also check nil signee or signature in default verifier implementation.

[codecov]

Codecov Report

Merging #154 into develop will increase coverage by 0.04%.
The diff coverage is 96.25%.

@@             Coverage Diff             @@
##           develop     #154      +/-   ##
===========================================
+ Coverage    76.74%   76.78%   +0.04%     
===========================================
  Files          146      146              
  Lines        10890    10909      +19     
===========================================
+ Hits          8357     8377      +20     
+ Misses        1845     1843       -2     
- Partials       688      689       +1

[codecov]

Codecov Report

Merging #154 into develop will increase coverage by 0.17%.
The diff coverage is 82.75%.

@@             Coverage Diff             @@
##           develop     #154      +/-   ##
===========================================
+ Coverage    76.59%   76.77%   +0.17%     
===========================================
  Files          146      146              
  Lines        10893    10903      +10     
===========================================
+ Hits          8344     8371      +27     
+ Misses        1858     1842      -16     
+ Partials       691      690       -1

[zeqing-guo]

LGTM

[auxten]

Looks good