Fix ledger context leaks in `poc_receipts_v1` and `rewards_v2`

[xandkar]

Part of the bigger memory leaks hunt during resync.

Methodology

1. Spotted ETS table growths using beam_stats.
2. Instrumented:
   • blockchain_ledger_v1:new_context/1 and
   • blockchain_ledger_v1:delete_context/1 to log the generated table references and stack traces.

3. Processed the resulting logs to find origins (i.e. stacktraces) of references that did not have a corresponding delete_context call:

   $ grep -IHn 'Ledger context' miner/_build/resync_test/rel/miner/log/warning* \
   | awk '
   {
       op = $7;
       ref = $8;
       stack = $11;
   
       if (op == "DELETED")
           del[ref] = stack;
       else if (op == "CREATED")
           new[ref] = stack
   }
   
   END {
       for (ref in new)
           if (!del[ref])
               leaks[new[ref]]++;
       for (stack in leaks)
           printf "%d %s\n\n", leaks[stack], stack
   }'