This backport includes changes from the "loading_shared_values and size limited and evicting prepared statements cache" series,
"missing bits from loading_shared_values series" series and the "cql_transport::cql_server: fix the distributed prepared statements cache population"
patch (the last one is squashed inside the "cql3::query_processor: implement CQL and Thrift prepared statements caches using cql3::prepared_statements_cache"
patch in order to avoid the bisect breakage).
* 'branch-2-0-backport-prepared-cache-fixes-v1' of https://github.com/vladzcloudius/scylla:
tests: loading_cache_test: initial commit
utils + cql3: use a functor class instead of std::function
cql3::query_processor: implement CQL and Thrift prepared statements caches using cql3::prepared_statements_cache
transport::server::process_prepare() don't ignore errors on other shards
cql3: prepared statements cache on top of loading_cache
utils::loading_cache: make the size limitation more strict
utils::loading_cache: added static_asserts for checking the callbacks signatures
utils::loading_cache: add a bunch of standard synchronous methods
utils::loading_cache: add the ability to create a cache that would not reload the values
utils::loading_cache: add the ability to work with not-copy-constructable values
utils::loading_cache: add EntrySize template parameter
utils::loading_cache: rework on top of utils::loading_shared_values
sstables::shared_index_list: use utils::loading_shared_values
utils::loading_cache: arm the timer with a period equal to min(_expire, _update)
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