Fix for the keystore DoS vulnerability

[Shashank-In]

#195

[Shashank-In]

Test cases are failing because of the char limit I added.

--- FAIL: TestServiceCreateUser (0.00s)
652    service_test.go:47: CreateUser call rejected due to username or password exceeding maximum length of 25 chars
653--- FAIL: TestServiceCreateDuplicate (0.00s)
654    service_test.go:150: CreateUser call rejected due to username or password exceeding maximum length of 25 chars
655--- FAIL: TestServiceUseBlockchainDB (0.00s)
656    service_test.go:190: CreateUser call rejected due to username or password exceeding maximum length of 25 chars
657--- FAIL: TestServiceExportImport (0.00s)
658    service_test.go:230: CreateUser call rejected due to username or password exceeding maximum length of 25 chars

[swdee]

Changing it to a maximum length of 25 would prevent a large group of people from using their email address as a username which is quite commonly used.

[Shashank-In]

Closing as this is not the correct fix