Fix coding rules violations in walreceiver.c

Fix coding rules violations in walreceiver.c

1. Since commit b1a9bad9e744 we had pstrdup() inside a
spinlock-protected critical section; reported by Andreas Seltenreich.
Turn those into strlcpy() to stack-allocated variables instead.
Backpatch to 9.6.

2. Since commit 9ed551e0a4fd we had a pfree() uselessly inside a
spinlock-protected critical section.  Tom Lane noticed in code review.
Move down.  Backpatch to 9.6.

3. Since commit 64233902d22b we had GetCurrentTimestamp() (a kernel
call) inside a spinlock-protected critical section.  Tom Lane noticed in
code review.  Move it up.  Backpatch to 9.2.

4. Since commit 1bb2558046cc we did elog(PANIC) while holding spinlock.
Tom Lane noticed in code review.  Release spinlock before dying.
Backpatch to 9.2.

Discussion: https://postgr.es/m/87h8vhtgj2.fsf@ansel.ydns.eu