Fix various memory leaks

[mreynolds389]

Issue Description

This was all on main branch

• Display attributes were leaked on searches
• Work thread indexes were leaked at shutdown
• Accept thread was leaked at shutdown
• When we reused a search pblock, we did not free the "operation" and "search attributes" before overwriting them with freshly allocated structures.

[mreynolds389]

63000bf..a548fa4 389-ds-base-3.0 -> 389-ds-base-3.0
7acdd65..2b68afc 389-ds-base-2.5 -> 389-ds-base-2.5
bf8a8f3..3aa6e3b 389-ds-base-2.4 -> 389-ds-base-2.4

[mreynolds389]

Might have a regression, heap-use-after-free from freeing operation in slapi_search_internal_set_pb() ...

[mreynolds389]

Might have a regression, heap-use-after-free from freeing operation in slapi_search_internal_set_pb() ...

Fixed in #6445