Restructure unixODBC related code

Zabbix interface for unixODBC library has following flaws:

• There are two places where unixODBC headers are (indirectly) included:
  • src/libs/zbxdbhigh/odbc.c
  • src/zabbix_server/poller/checks_db.c
• File odbc.c is compiled as a part of libzbxdbhigh.a but has its own header include/zbxodbc.h
• ODBC handle structure serves both as handle and result structure

We should either

• make a standalone unixODBC lib with it's own include/zbxodbc.h, src/libs/zbxodbc, libzbxodbc.a and hide all unixODBC details there
• or move all related stuff from src/libs/zbxdbhigh/odbc.c to src/zabbix_server/poller/checks_db.c and remove include/zbxodbc.h because it is not used anywhere else (we do similarly with netSNMP, OpenIPMI and libSSH2)