Replace netmgr per-protocol sequential function with a common one

Previously, each protocol (TCPDNS, TLSDNS) has specified own function to disable pipelining on the connection. An oversight would lead to assertion failure when opcode is not query over non-TCPDNS protocol because the isc_nm_tcpdns_sequential() function would be called over non-TCPDNS socket. This commit removes the per-protocol functions and refactors the code to have and use common isc_nm_sequential() function that would either disable the pipelining on the socket or would handle the request in per specific manner. Currently it ignores the call for HTTP sockets and causes assertion failure for protocols where it doesn't make sense to call the function at all.

Closes #2787 (closed)