What
===
Fetch the Ids for all search results then fetch the transaction details
one page at a time. TransactionGateway.Search should be backwards
compatible. The only difference being additional fields in
TransactionSearchResult and more than 50 transactions in the
Transaction field.

Per
https://developers.braintreepayments.com/reference/general/searching/search-results/java
up to 20,000 transactions can be returned.

Much of this work is based on the work by @rkulla:
https://github.com/dailyburn/braintree-go/commit/d44aa131caad6055c0e4aac2f3a24450aa818c63

Why
===
The Braintree API only returns 50 transactions with all details at one
time. To fetch all results from a query, the transaction ids must be
fetched first, then the details can be fetched by specifying 50 ids per
API call. This issue was first raised in braintree-go#112.