SKALE-4996 Add reentrancy guard for postIncomingMessages

[yavrsky]

1. Add reentrancy guard for postIncomingMessages
2. Move incrementing incomingMessageCounter before loop.

[codecov]

Codecov Report

Merging #1054 (3fd0fd8) into develop (18435c7) will decrease coverage by 0.03%.
The diff coverage is 83.33%.

@@             Coverage Diff             @@
##           develop    #1054      +/-   ##
===========================================
- Coverage    85.66%   85.62%   -0.04%     
===========================================
  Files           24       24              
  Lines         1074     1078       +4     
  Branches       288      289       +1     
===========================================
+ Hits           920      923       +3     
  Misses          64       64              
- Partials        90       91       +1     

Impacted Files	Coverage Δ
proxy/contracts/schain/MessageProxyForSchain.sol	68.96% <80.00%> (+0.44%)	⬆️
proxy/contracts/mainnet/MessageProxyForMainnet.sol	76.27% <100.00%> (ø)

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