Improve migration speed by using same connection in a LOB worker. (#533)

Presently, each LOB worker creates a new connection for every object copy,
impacting migration performance, especially with a large number of objects.
The fix here is simple: use the same connection within each worker.

However, while doing so, we encountered an unexpected error:

`BUG: call to pgsql_commit() without holding an open multi statement
connection`

Upon investigation, we discovered that the `connectionStatementType` was
set to an unusual value of 32767. In GDB, it was
(PGSQL_CONNECTION_MULTI_STATEMENT | unknown: 32766). This happens only
after the call of `msgrcv` function. There are a few theories; one suggests
that because both `msg` and `dst` are on the stack, `msgrcv` is overwriting
`dst` data when it should only change `msg`. Despite having
`-fstack-protector` enabled, we did not observe a "stack smashing detected"
error, so maybe this theory is incorrect.

In any case, moving `msg` to the heap seemed to resolve the issue. However,
it's important to fully understand this problem, as it may have
implications in other areas of the code as well.