Investigate Pending txs increase in 1.4

[hbandura]

There has been an increase in the pending txs amount since the 1.4 update. It seems to be steadily rising so it merits at least a time-boxed investigation.

As @jcortejoso wrote:

I think Mariano and me observed today is that the number of pending transactions in v1.4.0 
has increased notably, compared to previous releases: (first image is for v1.4.0 nodes, 
and second for v1.3.2 nodes).

Can anyone think on any reason or change that could have caused this?

@gastonponti added:

mmmm, maybe this PR: https://github.com/ethereum/go-ethereum/pull/21300

That was merged in the 1.9.18 version (between the 1.3.2 and 1.4.0, 
we added the 1.9.16-1.9.19 versions from upstream)

link: ethereum/go-ethereum#21300

[mcortesi]

Discovered it was related to the miniming gas price requirement. Tx with gas price cero were being accepted

#1749