Add support for connection pooling
* Add support for connection pooling to the core code.
* Add support for PostgreSQL connection pooling with tests.
* Add support for SQLite3 connection pooling with tests.
* Add support for MySQL connection pooling with tests.
* value: add missing includes
* mysql, sqlite3: use explicitly *out* stringstreams for serialization
* postgresql: fix: use max_digits10 instead of digits10
* detail: add float_safe_ostringstream, ensuring floats are serialized with enough precision
* mysql, postgresql, sqlite3, tests: fix: set float-precision for all streams
* run clang-format on changed files
* inline remove_cvref.h
* replace unspecific "wrapper" with concrete type name
* tests: split connector specific tests out of serialize/Float
