Fix ModbusBridge request/response logic

[dwrobel]

• Fix cases where the subsequent Modbus packet   can be send to the serial port (triggered either by   'ModbusSend' command or request from TCP bridge)   before an answer was received to the previous packet.

  This can happen in a setup where simultaneously:   - two (or more) modbus TCP clients are sending requests
through the modbus-proxy [1] to Tasmota,
  - ModbusSend commands are executed (e.g. using Berry).

  Log excerpt (from build with TASMOTAMODBUSDEBUG enabled):   14:51:18.940 MBS: Serial Send: 04 03 01 00 00 09 84 65   14:51:19.054 MBS: Serial Send: 04 03 10 0A 00 05 A1 5E   14:51:19.136 MBS: Serial Received: 04 03 0A 00 00 00 D0 00 00 01 AB 00 00 89 62

  Fix adds 'waitingForAnswerFromSerial' flag which is set after   we send data to the serial port and prevents sending another   requests before we receive an answer or timeout happened.

Fix stores temporarily a 'ModbusSend' command data and tries
to execute it after Modbus response has been received or
timeout has happened.

• Add 'ModbusSerialTimeout' command which sets timeout in [ms] for how long we will be waiting for an answer from the client device.   Default value is 1000 [ms] and it is not restored after reboot.

• Sends error 11 (0xB) (as TCP response) when no answer was received   from the serial port within the timeout set by 'ModbusSerialTimeout'
  command.

• Add Modbus 'TransactionId' to the logging.

[1] https://github.com/tiagocoutinho/modbus-proxy

Description:

Related issue (if applicable): fixes #

Checklist:

• The pull request is done against the latest development branch
• Only relevant files were touched
• Only one feature/fix was added per PR and the code change compiles without warnings
• The code change is tested and works with Tasmota core ESP8266 V.2.7.7
• The code change is tested and works with Tasmota core ESP32 V.3.0.4
• I accept the CLA.

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