Remove `tx_history` and `ledger_header` methods from API version 2.
Update `RPC::Handler` to allow for methods (or method implementations)
to be API version specific. This partially resolves#4727. We can now
store multiple handlers with the same name, as long as they belong to
different (non-overlapping) API versions. This necessarily impacts the
handler lookup algorithm and its complexity; however, there is no
performance loss on x86_64 architecture, and only minimal performance
loss on arm64 (around 10ns). This design change gives us extra
flexibility evolving the API in the future, including other parts of
#4727.
In API version 2, `tx_history` and `ledger_header` are no longer
recognised; if they are called, `rippled` will return error
`unknownCmd`
Resolve#3638Resolve#3539
