Add hash_remove and use in transp to fix #261

[balgillo]

Fix for #261

• Add a new hash_remove function which works like hash_flush but for a single hashtable entry.
• Remove mem_deref calls from transp.c and use hash_remove in close_conn instead.
  • This ensures that when the conn has already been removed from the hashtable, e.g. by hash_flush(sip->ht_conn),
    there is no extra mem_deref in transp.c.

Testing done

• Compiles fine
• retest -r passes
• Tested within two proprietary, internal test suites written in Python
• I have not reproduced this issue in pure C code due to the lack of request tests in retest.