Remove old receiver buffer implementation

[maxsharabayko]

The new receiver buffer implementation was introduced in v1.5.0. However, the old implementation is still available and can be enabled using the ENABLE_NEW_RCVBUFFER=OFF build option. It is a fallback solution in case some critical issue around the new implementation arises after the release.

The old implementation can be removed in the future major release. This would also allow changing the type of the CUnit::m_iFlag to bool, as the new receiver buffer tracks the state by itself.

TODO

• [ ] Remove the old CRcvBuffer implementation.
• [ ] Remove the ENABLE_NEW_RCVBUFFER build option.
• [ ] Potentially change CUnit::m_iFlag to bool.

Closes #1255.