sl: use global variable for event route index for local response

- rt should have been no longer used after previous commit
- reported by Ovidiu Sas
pdb: fix warning dereferencing type-punned pointer will break strict-aliasing rules [-Wstrict-aliasing]
presence_conference: fix what it looks like a copy/paste error
dmq: code cleanup, remove redundant includes
gzcompress: new module to compress/decompress SIP message body using zlib