Fix connection parsing in SOCI Firebird backend to handle spaces in values.

This commit rewrites `explodeISCConnectString()` function to properly handle spaces in the values so that connection strings such as `firebird://user=UID password=PWD service="c:\Program Files\foo.fdb"` work correctly (before we tried to open `"c:\Program` as a file).

I don't know what is the exact syntax of SOCI connection strings supposed to be, in particular whether there is any quoting, so I didn't do anything fancy and just made the minimal changes necessary to make the paths with spaces in them to work.