Allow external connection pool classes to be loaded automatically

This is similar to earlier changes for external plugins, extensions,
and adapters.  Just put your custom connection pool class in
sequel/connection_pool/my_pool_class.rb in the ruby load path,
make sure it registers itself in
Sequel::ConnectionPool::CONNECTION_POOL_MAP and then use
:pool_class=>:my_pool_class Database option.

While here, fix small RDoc issue.
Add Sequel.def_adapter_method as a method that evals code
Remove proc activation for Sequel.synchronize on MRI
Make Database#each_server raise if not given a block

Without a block, this would be a no-op that leaks Database
references, so make that an error.
Minor code formatting fixes
Minor RDoc fix
Minor cleanups in Database.connect

Simplify the connection option code and the connection block
return value code.

Set adapter scheme should always be called with a symbol.  With
the previous code, the ADAPTER_MAP would work if given a string,
but the @scheme would be incorrect.  All internal adapters use
a symbol, and external adapters should as well.