Fix connection leak when requested object file does not exist on disk

If an RDS format filehash is created on disk and an object that does not exist
is requested then there is a connection leak. If this process is repeated >124
times then R runs out of connections (which is of course bad news).

One safe approach appears to be:
* first create connection
* then open it for reading (potentially resulting in error condition)
* always close it (destroying connection) even when erroring out

The problem can be demonstrated as follows:
```
dbCreate("mydbRDS", "RDS")
db <- dbInit("mydbRDS", "RDS")
dbInsert(db, "a", 1:10)
showConnections(all=T)
dbFetch("a")
showConnections(all=T)
dbFetch(db, "nonexistentkey")
showConnections(all=T)
dbUnlink(db)
```
And the exact origin can be seen in these two snippets

```
# Leaks a connection
showConnections(all=TRUE)
con <- tryCatch({
    gzfile(tempfile(),'rb')
}, condition = function(cond) {
    cond
})
showConnections(all=TRUE)

# No connection leak
showConnections(all=TRUE)
val <- tryCatch({
    con<-gzfile(tempfile())
    open(con,'rb')
    readLines(con)
}, condition = function(cond) {
    cond
}, finally = {
    close(con)
})
showConnections(all=TRUE)
```