Fix reusage of proxy_l structures.

As proxy_l structures contain information about the DNS-based failover (iteration in he struct, which SRV or NAPTR records were used or not), such structure cannot be used more then once for sending a message out.
Fixes:
- do not use the same proxy_l for multiple functions in script (remove the add_proxy() and find_proxy() )
- avoid using the same proxy_l structure for all the calls of same function (when proxy_l is a function parameter)

Fixes #224
Many thanks for Chris Maciejewski ( @l7s ) for support on troublehsooting this.

(cherry picked from commit f7f2cb41d6f562d0c3d6748a827d55e12cb05174)