Disable uncache_aggressiveness with allow_mmap_reads

Summary: There was a crash test Bus Error crash in
`IndexBlockIter::SeekToFirstImpl()` <- .. <-
`BlockBasedTable::~BlockBasedTable()` with `--mmap_read=1`, which
suggests some kind of incompatibility that I haven't diagnosed. Bus
Error is uncommon these days as CPUs support unaligned reads, but are
associated with mmap problems.

Because mmap reads really only make sense without block cache, it's
not a concerning loss to essentially disable the combination.

Test Plan: watch crash test

include/rocksdb/options.h

@@ -388,6 +388,8 @@ struct ColumnFamilyOptions : public AdvancedColumnFamilyOptions {
   // block cache entries (shared among copies) are obsolete. Such a scenerio
   // is the best case for uncache_aggressiveness = 0.
   //
+  // When using allow_mmap_reads=true, this option is ignored (no un-caching).
+  //
   // Once validated in production, the default will likely change to something
   // around 300.
   uint32_t uncache_aggressiveness = 0;

table/block_based/block_based_table_reader.cc

@@ -137,7 +137,13 @@ extern const std::string kHashIndexPrefixesMetadataBlock;
 
 BlockBasedTable::~BlockBasedTable() {
   auto ua = rep_->uncache_aggressiveness.LoadRelaxed();
-  if (ua > 0 && rep_->table_options.block_cache) {
+  // NOTE: there is an undiagnosed incompatibility with mmap reads,
+  // where attempting to read the index below can result in bus error.
+  // In theory the mmap should remain in place until destruction of
+  // rep_, so even a page fault should be satisfiable. But also, combining
+  // mmap reads with block cache is weird, so it's not a concerning loss.
+  if (ua > 0 && rep_->table_options.block_cache &&
+      !rep_->ioptions.allow_mmap_reads) {
     if (rep_->filter) {
       rep_->filter->EraseFromCacheBeforeDestruction(ua);
     }