reduce memory overhead of CTX_TABLE_ENTRY

reduce memory overhead of CTX_TABLE_ENTRY

We don't need to allocate entries for the ctx table.  we can just make
it a sparse array of void* values, and set the thread-local data
directly.  This saves on allocation and cleanup costs

Reviewed-by: Saša Nedvědický <sashan@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/27794)