Inside namespace utils, unqualified coroutine:: resolves to the
utils::coroutine class (utils/coroutine.hh) rather than the
seastar::coroutine namespace. This causes build failures when
replica/database.hh is added to the precompiled header, because
utils/coroutine.hh becomes transitively visible in all TUs.
Qualify all coroutine:: references with seastar:: in affected files
under utils/ and db/.
This patch extend `lru_string_map` with `sized_string_map` - a class that helps to control cache size.
It implements cache resizing in background thread.
Adds a lru_string_map definition.
This structure maps a string keys to templated arguments, allowing efficient lookup and adding keys.
Each lookup (and adding) puts the keys on internal LRU list and the entires may be efficiently removed in a LRU order.
It will be a base for the expression cache in Alternator.
