fix: treat addresses as strings in sdk internals

[egonspace]

Description

We had a chronic problem with the high cost of marshal/unmarshal of Account, Balance. This is due to the address bytes <-> bech32 address type conversion.

Of course, we've alleviated this problem to some extent by caching the pairs of bech32 strings and bytes, but it's more preferable that this happens so frequently that it's essentially unnecessary than cache. In other words, this cost is not required if the address is stored as the string type. It is only necessary to convert string address of an incoming msg to bytes when validating. Alternatively, if we only validate by checking bech32 prefix and checksum without converting to bytes, we will be able to do so at a much lower cost. (This could be carried out in a separate PR later.)

Main changes:

• Addresses are all stored in string type in the store. A once-stored address does not need to be validated later. No bech32 encoding/decoding cost when we marshal/unmarshal it.
• Cannot typecast address bytes with AccAddress directly. -> We must convert using BytesToAcAddress() function.
• Unable to directly compare an AccAddress value and a ValAddress value. -> One AccAddressToBytes()ed must be compared to a ValAddress value that is a result of BytesToValAddress
• Ostracon still records validator address as byte.

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[codecov]

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[whylee259]

LGTM, The address validation will be discussed in another PR.