[PATCH] Check system cache invalidations before each command in transaction

Hello guys.
In the previous discussion [1] we find out that while we are in
transaction function definition is not invalidated if it was redefined
in another session. Here is a patch to fix this. Also, I did a small
perfomance impact measurement (test.sh in attachment) on my home PC
with Intel(R) Core(TM) i7-8700 CPU @ 3.20GHz. The result is (each
transaction is a 10 million calls to functions):

- without patch
latency average = 37087.639 ms
tps = 0.026963

- with patch
latency average = 38793.125 ms
tps = 0.025778

What do you think about it, guys?

[1] 
https://www.postgresql.org/message-id/flat/1205251664297977%40mail.yandex.ru